CFLs probably, as much wattage as you can fit and still keep the temps down. In your dimensions, which are the H W D? <--Or is it like that? If you have some cash you could line it with T5's.

Yes, you can do a lot to keep them lower and bushy within reason. Tie em down, Top em, pick known short strains to grow (LowRyder might be good for you) From a fat lowryder you might get 1/2 to an Oz and you could probably fit two maybe three in there.

When you are ready to flower, just switch - they don't mind a bit. (with Lowryder, no switching needed - they are auto flowering)

Good luck & happy growing