OK, Im a cruiser captian LT Cmdr 6. I have my 4 boffs, 1 sci, 1 tac and 2 eng. I have maxed all the trainable skills on them. One of the eng has been promoted to Lt.

Do I:

1. Train another to Lt in preperation for commander?
2. Grab another from the candidate pool and start training him/ her also for commander?

Im a bit confused what to spend the bo points on now.

You can actually have BO's in a pool trained for different circumstances, as the game says "Dont be afraid to swap BO's for different situations"

I would personally just keep em, no rush to spend them really

Also, if you promote a BOff to Lt, they can use all of their ground skills. Just because they don't have a 2-skill console in space doesn't mean they can't use the additional ground skills planetside.

I always promote everybody. It costs Merit, but that is readily available.

And those additional ground skills can be very helpful in combat.

ETA: Another possibility to consider is to wait until you get more requests by other Star Fleet officers to transfer to your ship (the little "mission" that gives you a new BO as a reward every few levels). Some of those new BOs have great traits that help out ground combat, so you might want to keep slots and training points open for them.