Here are the rules. For perks you can use any combination you want but you CANNOT change their teir. For example you cannot take the MW2 Marathon and move it to the 3rd teir like it was in in BO. Also no perks from BO2 simply because we have not tried any yet. You can have 5 teir one and teir two perks and 6 teir 3 perks. Also the pro versions are the same as they were in that game.
Now if you want to go the extra step you can do guns with the same rules as above. Any gun from any CoD with no changes. Now here is MY perk system. Feel free to disagree if you don't like mine. As always keep this mature and no hating on other peoples. Also feel free to do any perk from any teir from any game if you really don't want to do as i said above.