I had a shark like that, had to rehome him to someone with 125 gallons. I wouldn't recommend smaller than 75 gallons to keep them really happy and healthy. Otherwise, the aggression level, which increases as they mature, can wipe out your tank. Mine ate all of my danios.
As for the mollies, I'd recommend 20 gallon minimum.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~