Your bala sharks will eventually outgrow your 55g as you know....
Tiger barbs are nice in a school, you might want to add 2 or 3 more. They are fairly hardy, as I understand, but like all schooling fish are better in a group.
I'd get ridd of the balas and get 1 (only one) ruby/ rainbow shark, and something like a green severum cichlid, or 2 yellow labdichromis to complete your semi-aggressive tank.
A couple of catfish like Hoplo catfish with those, would work too. Just my 2 cents.