If you want a rainbow shark or red tail black shark, around 30 gallons is the minimum. If you want two, 100 gallons probably won't be enough, but it might. They claim very large territories for their size. Once they're mature, they won't tolerate anything that looks much like them in their territory, and sometimes become aggressive towards other fish (RTBS are more notorious for this than rainbow/red finned sharks).
Bala or silver sharks are quite different - they're a large shoaling fish that needs a group and a very large tank. - 75 gallons and up. They're cute little guys in the stores, but don't be fooled - they become cute foot-and-a-half guys.
I'd suggest a 40 gallon, really - with that, you could have a shark and a reasonable shoal of tiger barbs (Attractive semi-aggressive shoaling fish), at least 6-8 to keep them from terrorizing tankmates too much. Any robust nimble fish would work with them, avoid long fins. Most tetras would be suitable. Many larger bodied tetras are nearly as aggressive as the tiger barbs - black phantoms, serpaes, black widows. Some smallish cichlids would be good as well - kribensis for example. Avoid the really aggressive ones like convicts (though I have heard that convicts and tiger barbs can mix safely) or anything big enough to eat the other fish.
I'd be careful with the lobster with fish. It does often work, and blue lobsters make beautiful additions to most any tank, but the crayfish is likely to eat any fish it can catch. A shark would be the most likely target, being a bottom dweller.
