They are compatable but will need a large tank, balas grow to around 13" and SDs get to around 6" round, the tank will need to be at least 75 gallons really.
ok, cewl. as long as they're compatible its all good. tnx cfc. BTW one last thing, can I add add 1 rainbow shark to a tank with 2 silver sharks and 2 silver dollars?
Adding a rainbow shark will be fine, agressive sharks like redtails and rainbows only attack others of their own kind and fish that have a similar appearence (SAEs for example)