ToME: the Tales of Maj'Eyal

Just wondering if an item my class doesn't use like a ammo pouch for a summoner for example adds to stats if i equip it or does it have to be usable by my class.

It adds, but since it adds stats that only relate to ranged weapons, it will never be meaningful unless you are using a ranged weapon.

Yeah, ammo items won't help you if you aren't using it for ranged attacks.