First one needs to think about what such a military would be facing -- and that's probably going to mean being outclassed in both tech and tonnage. The solution in this situation is always "Quantity has a quality all its own" so you need 'mechs that can work together to equal larger 'mechs. This means throwing lances at one or two heavies being the norm, entire companies needed to face off against enemy lances.
So one needs to think like the US or Soviets during WWII. Where the OPPOSITE of the BattleTech mindset comes into play. In BT there's this noodle doodle nonsense about the "expense" of training warriors and how they're a "warrior elite" like medieval knights. That thinking needs to be thrown in the trash in favor of what Zhukov once said: Meat is cheap.
Much akin to the old German tanker joke, where "One of our Leopards is worth ten Shermans, but these blasted Americans always have that eleventh Sherman."
The next concern is logistics - supplying with ammo, the expense of training and providing qualified pilots, parts supply train and commonality. As such the designs should all likely be the same weight and use the same engine with only rare exceptions such as specialized forces; aka scouts.
Something the Clans figured out with Omni's
Some equipment -- like machine guns perhaps -- would be required on most platforms for "peacekeeping" (riot breaking) during "peacetime". Likewise utility aspects -- like articulated hands -- would also have to be "essential" to all platforms so that they're "hangar queens" when stood down from actual warfare.
Likewise there's some stuff that's bulky and not as effective in "introductory".. to that end I would NOT use any of the autocannon. Missiles would be a necessary evil and generally provide more bang for the buck. I'll stack a LRM-15 against a AC-10 any day.
Take a lesson from the USMC -- every soldier is a rifleman FIRST.
Since we're basically talking GI (General Issue), I would likely field the vast majority of units as:
Title: Bagramyan
Chassis: 30 tons, 4/6/0, 6.5 tons armor, 11 standard sinks
Armament: PPC, 2xML, 2xMG@0.5
Cost: 2,098,440
BV: 749
Deployment: 1 in 2
Two of which gives you the long range firepower of a Warhammer, mix in a third they could probably go toe to toe with a Marauder II. Two of these would have superior heat profiles to a single whammy if range bracketing is used properly.
For fire support, we just swap the PPC, sink, and one ML for a LRM-15 with two tons of ammo:
Title: Konev
Chassis: 30 tons, 4/6/0, 6.5 tons armor, 10 standard sinks
Armament: LRM-15@2, ML, 2xMG&0.5
Cost: 2,071,340
BV: 655
Deployment: 1 in 4
This design should hang back and use indirect fire when possible, laying mines and providing cover for the grunts in Bagramyan's to move up.
Likewise an in-fighter may be wanted for more urban versatility and late battle oomph. Let the PPC's on the rank-and-file open up some holes, then exploit those holes with SRM's just as a LB-20x would on more advanced units.
Title: Vatutin
Chassis: 30 tons, 4/6/0, 6.5 tons armor, 11 standard sinks
Armament: 6xSRM-2@2, 2xMG@0.5, flamer
Cost: 1,876,190
BV: 568
Deployment: 1 in 6
Six SRM-2's gives you a good sanding capacity, and I chose -2's because statistically more smaller launchers are likely to do SOME damage sooner, as well as opening the door to specialty ammo such as inferno. In terms of cost and BV, I'd take this over the best Urbie any day.
From there you need a scout. Scouts SHOULD be expensive, they should NOT be used as anything more than a harasser from long range, and they will violate the "same engine" policy.
Title: Khudyakov
Chassis: 30 tons, 7/11/7, 4.5 tons armor, 10 standard sinks
Armament: LRM-5@2
Cost: 2,797,540
BV: 580
Deployment: 1 in 12
The tactics for this unit is again to SCOUT, not engage, finding the enemy, using it's superior mobility and cover to at BEST act as a harasser. It should stay in its medium range using its maximum jump to avoid being hit, plinking the enemy with the massive amount of ammo it carries as an annoyance. You don't march your lightly armored fast movers up to point blank range.
Plink plink, plink plink, his missiles went plink plink plink... While piloting my Jenner K, what to my surpise... a little Clan Dasher was following me, almost half my size.
Note my deployment figures. A "standard" company would field 6 Bagramyans, 3 Konev, 2 Vatutin, and a single Khuyakov. In field use such a light company would be expected to be put against a heavy or even assault lance of "tournament legal" tech. Just as the reds and US army did the Germans.
... because they're all cheap to make; for the cost of a single Victor or Thug, you can get an entire lance. Hell their uniform small size could be used to standard all of a military's dropships to double-stack them (which was done in one of the books haphazardly...) further enhancing deployability and possibly making up for their overall lack of speed.
Beyond the 'Mechs I would certainly supplement with vehicles. Proper artillery units and LRM carriers topping the list. Depending on the loyalty and mindset of the faction, I might also consider deploying some of the classic "one shot heavy guass" hovercraft / Baka bombs.
Title: Durak
Type: Hovercraft
Chassis: 20 tons, 13/20/0 ICE, 1 ton armor, single crew
Armament: None
Cost: 193,667
BV: 17
Technically a "desperation weapon" you still gotta love that a charging attack from one of these would be equal to a ballistic hit that rolls on the kick table... and with that type of speed you can easily push it up into the 20+ points of damage range.
I would also consider perhaps breaking the "30 ton only" rule for command units. Unit commanders are better trained, a valuable resource, and should be in heavily armored units. At the same time I would restrict their firepower to similar to the light units so that said commanders have the presence of mind to stay the hell back, and to keep parts and ammo commonality.
Something like:
Title: Zhukov
Chassis: 60 tons, 4/6/0, 12 tons armor, 15 standard sinks
Armament: PPC, LRM-15@2, 2xML, 2xMG@0.5
Cost: 4,922,260
BV: 1,306
Deployment: 1 in 36 (company commander)
Violates the overall concept, but reserved for company commanders so very low production numbers overall. Consider it the "high" in a "high low mix". Whilst not maintaining chassis compatibility, it at least chooses from the same weapons loadout as our top two other designs.
If operating in the "modern" era where you're facing off against more advanced opponent I would design from the START for these to have field kit upgrade paths to more recent gear. That means all designs should be "ready" to swap to FF armor, double sinks. The PPC equipped could then easily go with the ER flavor as a drop-in, but I'd retain the medium lasers and use the excess mass to add more of them. The Vatutin would get the most changes, swapping the standard-2's for fewer streaks so as to conserve ammo. Again, Streaks are a logicians dream.
Hence the fieldkit upgrades would go something like this:
Title: Bagramyan (Clan Invasion Field Kit)
Chassis: 30 tons, 4/6/0, 5.5 tons FF armor, 10 Double Sinks
Armament: ER PPC, 4xML, 2xMG@0.5
Cost: 2,466,340
BV: 868
Title: Konev (Clan Invasion Field Kit)
Chassis: 30 tons, 4/6/0, 6 tons FF armor, 10 Single Sinks
Armament: LRM-15@2, ML, 2xMG&0.5, CASE
Cost: 2,207,840
BV: 712
Title: Vatutin (Clan Invasion Field Kit)
Chassis: 30 tons, 4/6/0, 6 tons FF armor, 10 Double Sinks
Armament: 2xStreak-4@1, 3xML, 2xMG@0.5, CASE
Cost: 2,390,340
BV: 765
Title: Zhukov (Clan Invasion Field Kit)
Chassis: 60 tons, 4/6/0, 11 tons FF armor, 10 Double Sinks
Armament: ER PPC, LRM-15@2, Streak-6@1, 2xML, 2xMG@0.5
Cost: 5,648,260
BV: 1,520
Reasonable field kits that should have low turnaround times that would not significantly impact the supply chain. Biggest struggle would be Streak ammo production.
In general I like the overall idea of this "challenge" as it's something that's just plain screwball with IS militaries. Whilst playing a wide variety of units is cool and fun, it's not realistic for any of the House militaries to be doing. REAL militaries usually have massive numbers of "grunt" units that are all the same, and for very good reasons. If one were to take c-bill cost and the overall logistics into account, a force equipped in this manner would have far higher "operational readiness" and handle the wear-and-tear of a prolonged conflict in a far better manner than a military whose TOE is spit out by the random unit tables for any IS faction. A good parts stockpile will kick the backside of "field recycling and salvage" any day in terms of reliability, time, and effort.