rtbs's all have different personalities. most are quite aggressive, but some are not. even if yours isnt aggressive, it will need a larger tank, mininum 29 gallons. if you dont, the shark will be stunted in growth, and then he will die at a young age. i have heard of rtbs's that were very old, i think it was 6+ years! if you keep it in your tank, it will die at maybe 2 years, if lucky. so do the right thing and get him out of that tank.
dont put a pleco in a 10 gallon! i agree with mike, try ottos, they are supposed to be great fish.
not to be mean, but you really need to research before you get your fish. you cant just get anything you want, it wont work.