I would guess that in the mod's XComClassData ini, in the Viper template, let their allowed weapons include rifles as well,

+AllowedWeapons=(SlotType=eInvSlot_PrimaryWeapon, WeaponType="rifle")

Should work like that, if I recall correctly... This should let them use any tier of assault rifle (though I'm unsure if their model will break when firing or if they'll try to follow the human firing model...)

thanks, ill try it

I got this to work, the problem however is that it completely breaks the firing animation.
A workaround I wanted to try is to just manually buff the viper stock rifle once you researched the plasma techs ingame, however I couldnt find how.
Any ideas anyone?