1) A light flying drone should be unarmed. Its' benefit should be the flight and vision, not more weapons
2) UGVs have to be balanced, though. If they were super snipers, why would you take anything else? As they can't gain XP, they should have good enough stats (and upgradable via research), but not great ones
3) Multiple weapons types per platform are in the plan, loosely. Some of the weapons will likely get ammo upgrades too, as existing weapons do (EP rounds, AM rockets etc). It's probably better if a single UGV only mounts a single weapon at a time though.
All UGVs should be able to be damaged by man(?)-carried weapons though, at least heavier ones, otherwise combat degenerates into "who has the last tank left?", and they mop up
4) I think its better that all UGVs normally carry only one weapon. I suppose it might be possible to use the existing off-hand weapon mechanics to mount a second one, but it should be expensive and limited if it exists at all. Otherwise I forsee late game tactics using 3 overpowered UGVs and a handful of psychic troops to overrun every mission (my brother did something similar in the original XCOM, had 2 uber-psychers who took down alien bases using just them armed with fusion launchers, and the power of mind control. Needs luck to pull it off, but it's beautiful when it works)
5) It's doable with the ammo selection code now, but it would leads to issues quickly. The grenade launcher is a good example. It used to fire 1 or 3 shot bursts, with 4(?) firemodes for each (aimed timed/airburst, and snap timed/airburst I think). Very messy.
Now, a even strictly single shot cannon needs at least aimed and snap modes. Should it have a burst mode? A super-snipe mode for all its' TUs, but a slightly higher to-hit? Something else? And how many ammo types should it be able to carry? These aren't full on tanks, just RC cars on steroids, remember. That cannon tank is going to have like 8 shots, maybe. Simpler is probably better, for a lot of reasons
GLs are GLs, probably best to just call it a mortar, which it likely would be, anyway
I'd just say don't go too mad with the model making until you are fairly sure what is needed / might even be possible (it's a pretty old engine) / even fits in the game. A lot of decisions about what can and can't be done in the world have been made that aren't always obvious from the game as it stands