UM-R60 UrbanMech
UM-R60 UrbanMech
WSP-1A Wasp
STG-3G Stinger
STG-A5 Stinger LAM
WSP-100 or WSP-100A Wasp LAM, depending on munitions availability.
The Urbies are civil defense units that, thanks to the AC/10s and solid armor, can hit harder than their 30 tons normally allows. Because they're better at providing heavy support for infantry than massacring infantry or civvies, they seemed like good anchor units.
The WSP-1A, especially when loaded with Infernos, makes a great vehicle hunter but, as Worktroll pointed out, SRMs are versatile enough if you ever need to use it for antipersonnel or crowd suppression, too.
Had the STG-3P been available, I would have been tempted to simply go with ten of those but, in its absence, the STG-3G seemed like a good choice for a bug 'Mech that has the armor and firepower to make itself a total pain in the ass and cause chaos above what its 20 tons normally permits. If you prefer, a LCT-1E would do similar, but share fewer parts in common with the Wasp. Unlike the STG-3R, its lack of machine guns shouldn't cause unholy terror amongst your subjects (yes, I'm going for the enlightened, benevolent despot here).
On to the LAMs: the STG-A5 was an obvious choice, since it hits hard, has decent armor, and lacks ammo-based weaponry. It has a chance of being a not-awful fighter or BattleMech: I've got one in a MekHQ campaign in 2998 that, since MegaMek lacks LAM rules but has LAMs, acts like a Fire Javelin with a ton less armor, one less laser, and one less heat sink. It works surprisingly well, and even soloed an equal-skilled Enforcer in a duel. While it's equivalent to 60 tons, it can reinforce your Guardian-Bs while also reinforcing your BattleMechs.
I considered a second Stinger LAM for parts commonality, but the WSP-100 and WSP-100A Wasp LAMs offer capabilities that they don't. For the -100, you get a nice bomb truck that, because it can alternatively carry extra fuel, also serves as a long range aerospace patrol. If there are sufficient alt munitions available for that bomb bay, this can be a vicious choice when used carefully.
The WSP-100A, meanwhile, is a bit more conventional, but gives you something you would otherwise lack: LRM support, basically serving as a less-awesome Valkyrie, one that's missing a heat sink and two tons of armor. This could still work out OK on tabletop, even with MegaMek where you lose out on being able to convert.
Amusingly, the whole kit and kaboodle of 'Mechs would easily fit into a Leopard, which is a nice bonus. Alternatively, an Ares Mk I landing craft can carry 65 tons, allowing you to carry an Urbie and a BugMech in a single shuttle with 15 tons of supplies to spare: two shuttles escorted by your LAMs could've carried your whole force to your new homeworld.
Neat idea, Liam, and I think you just gave me my next MekHQ campaign.