Take the large launcher off, smooth out the top and then give him side cannons. Smaller ones that are fixed (maybe some elevation ability on a pivot point).
Then they could mount on either arm in robot mode. So it'd still look like Megs, but different.
Or have the two link together and mount to one arm.
This gives you more of a smooth top look (most APC's are smooth top, some have turrets but they're multi-purpose vehciles like the LAV and Bradley M2 and M3's).
As for others, Soundwave could be converted into an M577 APC as well, boxier, with tracks, but would give him a similar "box" shape in robot mode (
reference).
This would allow him to be protective of his little horde. They, in turn could become a whole slew of things. From military radios to crates to military type computers (large crate affairs that they carry in the 577's) that would, again, allow them to maintain their current transformation styles but have new forms befiting of Soundwaves.
Also, the 577 is a command and control vehicle, so it would be a good choice for your commo guy.
Another thought would be to use something like the LAV (
reference 1,
Reference 2) for the likes of Runabout and Runamuck. Sturdy vehicles that would fit their personas, which, when flipped, would have them acting wildly to protect others. Picking on close range version (small barreled turret) for close up protection and one long range version (long barrel or anti-aircraft equipted version) would allow for them to act crazy to protect the innocent.