Some brainstorming idea:
USF:
-Grenades cost -25mp -5f
-Officers (LT + Cpt) cost each -100mp but they only arrive with 1 model (the officer). At 1 model, they gain a "reverse" merge ability. They can absorb any Rifle squad, no xp is saved in the process. If at 5 models, they can be momentarily at 6 models till 1 of them dies.
This achieves several goals.
1- Reduces the power spike the moment the officer arrives on the field. You would had to spend mp + time to reinforce to 5 models.
2- You can ditch the unit (a single model) if you don't need it. You can reduce the amount of Rifles you have, opening space for other call in infantry units.
3- Getting both LT + Cpt is more accessible.
4- Cpt at 1 model means it can be used for supervise effectively, without wasting popcap in the base.
5- Officers should get both grenades types unlocked.
When you reduce the powerspike of the "free officer", you open up space for buffing other aspects of the faction which might lead to space for nerfs in other factions as well.
These are particularly good ideas that would provide a well-thought-out fix to the recent conundrums that USF faces. It also leans on existing game-play mechanics that are known in the game. Your idea, in addition to replacing REs with Assault Engineers (whilst keeping RE as start unit and making it doctrinal), would fully suit this army and generally make things better.