I was thinking - several weapons in UFO:AM use, for example, the same 9mm bullet (Uzis and a few pistols, for example); IRL, a Glock magazine could be loaded into an Uzi; M-16 magazines could be loaded in Galil and Tavor TAR-21 rifles.

So, is that possible to make several guns to draw from the same ammo pool?

Well, the laser and plasma weapons behave as you say. But you must really understand how these weapons are defined in the game if you wan't to do something similar for the rest.

Main problem is that no weapon defines it's max capacity, this is directly defined by the magazine, so you could easely end up giving small chamber guns large ammo clips.

Anyway the files you want within the gamedata.vfs file are:
tactical\configs\game\listofweapon.txt
tactical\configs\game\listofmagazine.txt
tactical\configs\game\listofammo.txt

This can be more problematic as transgenants also use human weapons and thus you will have to edit the equipsets as to prevent the game from crashing due to incoherent data.