I wouldn't mind working with this a big. If we could expand the available pool of voxel models.

The gun representations wouldn't have to be 'perfect'.
We could have:

Soldier with Rifle on back(for secondary usage)
Soldier with pack on back(reuse backpack model?) for when a deployable is in secondary slot
Soldier with tube on back(for rockets/tubes/mortar)
Soldier with long rifle on back(snipers)
Soldier with MG on back+belts(MGs)
Existing model for nothing on back(lack of inventory)

Those are some good ideas.

I was thinking (i guess for greens only):
Ghillie suits for snipers
Ammo belts as mentioned by OP for Machine guns
Maybe some form of stealth outfit for silenced weapons like a red beret or a black outfit
Steel pot helmet and tank top for Shotgunners
for riflemen you could keep them the same.

These apply to the primary weapon slots and combining each of these with a second model would mean you would have each primary model duplicated with the secondary attached. Also each file would take around 250kb which would significantly increase the size of the modpack.

It's a lot to take in and a lot to think about really. I'll play around a bit more to get a better understanding of modding. I don't even know XML.

If you mean the amount of disk space they take, I guess it's a minimal issue in the end. For distribution, the XML files tend to zip pretty well, in some cases down to magnitude of 5%.