There has got to somehow be a way of making the crew have identical popcap to the vehicle it came from.
Would be impossible. If you lose that vehicle, you'll end up with a 12 popcap squad. Placing pop cap on specific units would counter the idea of having tanks switch crews for experience.
The best idea is to have tank crews have those abilities but be tied to their vehicle and only it until its destroyed (the tank crew cant stray 5m away from its tank until its destroyed or something)