i think that u should be able to send your marines and all infantry really (including spartans) to the armory where u can select the squads weapons and choose how many of what u wouldjust have to pay a certain price for each weapon

like a squad starts with all assualt rifles and u send them to the armory and give 2 a Br 1 a rocket 1 a sniper and 1 a anti air rocket and let the rest keep their assault rifles and then u pay a certain amount for each weapon upgraded

 this could probly also work with armor