Lock veteran riflemen to cp 2-3, but make it consistently give vet 2 rifles. That way it can't be abused early game, and gives USF player easy to replace riflemen for late game. Nerf vet gain so it takes longer for them to get to vet 3 though than say a normal vet 2 riflemen. 20 more manpower for a squad that already has vet and is a call in is a steal.
^^^ Or something of the sort. It'll be better balanced for replacements later in the game than as an unfair advantage at the start of the game.