(1) It seems to me like the light weight class of IS units is pretty doomed against the Clans, due to the ease of hitting fast-moving units with Clan pulse lasers. So if an IS player wants fast units, they should probably choose fast mediums or heavies rather than light units that rely mainly on speed for defense. Does that sound right?
(2) Should you take as many long-range weapons as possible? Or give up on the long-range game and just try to close in to short range and fight the battle there?
(3) What about if you're trying to take on the Clans with 3025 tech?
(4) Just in general: what works against the Clans given their many advantages?
From someone who plays from a war-gaming perspective-
Addressing them in order of effect . . .
#4- Realize what your advantages are in any engagement against a pure Clan mech force- you will have numbers, armor & internals (mass) as well as overall/total firepower while the Clan player has speed, range, survivability and harder hitting weapons. The IS player also has a better combined arms option, especially when BA come into play for the IS. You also need to realize the Russian saying, 'Quantity has a quality all its own,' which means you will most likely outnumber the Clan player in mechs let alone total units- and use that advantage knowing you will lose equipment. Repeat after me, you will lose equipment. While the Clan force should have a speed advantage, you should still be able to pin them against some terrain feature on the map (besides the edge) since unless they mount JJs they do not have the speed to cross water, elevations or woods that quickly and still maintain LOS for engaging.
As a Clan player I always wanted the largest maps with the least restricting terrtain- no cliffs, 4 hex wide rivers, or great blocks of forest (3 or 4 hex wide, 10+ hexes long). I would run across the map with my Timberwolf Prime & E, Gargoyle Prime & D, Stormcrow Prime, Adder B & D, Ice Ferret A or anything else armed with long range weapons & speed. They would also be the mechs carrying Elementals. I would arrange my last move to put me behind a L1 hill or in woods at 22-25 hexes and drop the Elementals- hill was the best since I played on servers that had DB on, the IS player lost track of Elementals that way. And then my cavalry units would walk backwards firing the cERLL, cERPPC, LRMs, ATMs w/ER rounds and LB-5X at the most exposed enemies . . . be it a Caeser 4S, Catapult or other long ranged design that packed a punch but had the weakest armor- typically a IS XL design. The Gargoyle Prime would specifically try to plink any armor in opposing forces. Best luck I had with that set up was a Timberwolf E hitting a Caeser 4S at 27 hexes, getting a TAC on the HGR- electric explosion, and the mech dies without firing a shot. Backing up they would eventually rejoin any of the slower designs which had moved forward at their 4/6 pace- Nova Cat Prime & A, Warhawk A, Guillotine IIC, Orion IIC, etc. Those mechs would have been parked in woods waiting for the IS to 'chase' the cavalry units into range, and angled so they could also start walking backwards & get their +1 mod w/o problems. If I had artillery I would also be dropping that in front of the wave of IS machines plodding after me.
With that description of my ideal sort of fight as a Clan player . . . how to counter it specifically? Combined Arms. Armor filling the fire support (Gauss & LRM) and cavalry (Harasser, Regulator, Fulcrum) roles, artillery to drop rounds behind retreating Clan forces and into woods that would provide cover- or to drop smoke to obscure LOS, ASF/CF bombing runs of inferno & RLs, VTOLs flanking & sniping, BA to protect your flanks and deal with any Elementals dropped off in your advance.
#3- 3025 tech during the Invasion is just going to increase your numerical advantage and require pushing into terrain . . . the one other thing I would say here is if you CAN score some NARC beacons, then you can drastically increase your force's throw weight by loading your LRM & SRM equipped units with NARC ammo. The problem is a dearth of NARC launcher equipped cavalry units in 3050. Additionally, IMO you will want artillery vehicles on the board- things like the TAV & L1 Marksmen . . . it will clear out BA nests, hit sniper points and help herd the Clan forces.
#1- The old 3025 lights, yes they are moths in the flame if you get around the wrong type of mech . . . for example, no light mech ( or most mediums) should ever be within 20 hexes of a Rifleman IIC that does not have something else it will be shooting at as a absolute priority. A old Stinger or Wasp against Clan forces? Sure if I have to, but you are pretty much asking it to be scrapped when its fired at by something as a afterthought. Commando, Javelin or Locust? I can run them in as a backstabber under the proper circumstances but I still expect it to die most of the time- its a One Shot mech!
#2- Long range weapons are good, but remember yours are going to typically weigh more, have less range and not hit as hard. Your ideal range is going to be mid or close- places AC/10s, old PPCs, old Large Lasers, ERML, MLs and SSRM & SRM start hitting. Your weight of numbers will start taking affect at those ranges as you start getting more hits and from more directions.
Zell rules . . . remember the Clan pilot MAY challenge more than 1 opposing unit, and the reaction to armor/VTOLs/BA will depend on warrior/unit/Clan. And depending on the Honor Code rules the Clan player is following, if you started moving unengaged mechs or armor around behind my forces I would take that as aggressive action- it would edge you closer to being declared dezgra & open shooting. Clan players have debated what unengaged mechs can do or not do . . . can they provide a ECM bubble? What about sitting in woods hex to keep an enemy dueler from using them? What about shooting at woods to clear them? What about maintaining LOS on a enemy to allow a engaged ally to stalk their opponent from out of LOS in DB? Just what constitutes interfering in the duel?
Questionable behavior demonstrating you do not intend to honor zell can invite the Clan player to drop dueling before you are in position to exploit it.