OpenXCOM uses clip optimization, similar as XcomUtil. Unless I'm mistaken, all clips of the same type get combined : those fired by XCOM + those fired by aliens. So this may be too efficient, resulting in quasi-unlimited ammo for plasma weapons :
In a typical mission, aliens will take 1-10 shots of heavy plasma. Since a heavy clip has a capacity of 35, it means the player's team can already take 25-34 shots of heavy plasma "for free", because a clip would be lost anyway. You can have 14 soldiers taking 2 heavy plasma shots each, and it costs nothing. If you tried this in vanilla, you'd use 14 clips in one mission!
IMO, the standard loadout for aliens should be reduced. Even one heavy clip per alien should be enough. Because as it is now, it's quite impossible to need to build more plasma clips.