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Your shark by itself is too large for your tank, and they also prefer to be in schools. Trade it/sell it/move it.
The Platy is fine. Adding 3 more Neons to your school would keep them happy. Then you could choose a couple more small fish, or maybe a nice looking Dwarf Gourami to finish it off.
When I first started keeping fish, my lfs sold me and ID shark, within 2 months he got twice his size. I'm afraid that the others are quite right, he will need to go back to the lfs or you will need a very very large aquarium.
I agree with AshleyNicole, cories would be a good addition. They are very active bottom feeders and a lot of fun to watch. They will also help to keep the bottom clean. They like to be kept in groups so get 3 so that they have friends to play with.