Well, as Tactical captain in a Cruiser, the only grade III skills you can use would all be Engineering ones - because the Cruiser only has Lt. Commander and Commander slots for Engineers. You can't train them yourself, but rare/uncommon BOs with grade III skills can't be THAT hard to come by.

You'll have a more difficult time getting those skills versus an Engineer in a Cruiser, but it should be fully possible.

The other thing is that...even without the grade III's, you should have a balanced range of regular BO skills to choose from anyway. It's not like you'll be severely disadvantaged. Hardly. In fact you may end up with a more versatile selection of powers than someone who's specialized that way.