You can use recruit abilities that you haven't unlocked yet IF you happen to have a savegame where you DO have those unlocked.
For example: you can load a 100% completion savegame and select Lure. Then load a savegame where you only have the abilities Assassinate and Riot. Lure will be the selected automatically when you save the game. It's not stuck on this selection, so you can select Assassinate or Riot (or whatever others you might have) using the Assassin menu, but obviously you can't go back, since you haven't unlocked Lure in that selection wheel.
Anyway, I thought it would be good to share this. Can anyone take the time to test this on Playstation and Xbox?
I'll test it on 360 tomorrow morning, if I remember.