i wouldn't return the red tailed shark just because it is known to be aggressive. the tank is large enough for one red tailed shark to be ok in it. just closely monitor it to make sure that it doesn't get too aggressive. a lot of aggressive behavior can be solved by having more places to hide and having proper numbers of fish. in my experience with tiger barbs (have never had a buenos aries tetra) i've had zebra dano, neon tetra, khuli loach, dojo loach, batik loach, bala shark, rainbow shark, and neon blue dwarf gourami housed with tiger barbs. not all at once but i haven't had any issues with with housing any of the previous fish with tiger barbs. just stay away from long finned fish and make sure your you have enough of each type of fish.
if you are going to keep the red tailed shark do not add a rainbow shark.