Unfortunately there is unlikely to be a happy ending. Where you just can not do a larger tank I think you should steer clear for your own sake - this is going to give you heartache.
If they're tangs, and they're 4 - 5 inches, they're not shelldwellers which of course would be fine in the right numbers in a 29. I have a nasty feeling they might be tropheus. This will become one tropheus very quickly.
Almost as bad would be lamp. tetrocephalus/sexfasciatus...or any mix involving them...or any of the larger tangs...
As I say I would steer clear for your own sake.