Welcome to the forum!
With a new tank I would highly suggest that you take a look at this link before you continue buying fish
I just learned about cycling but I already have fish. What now?! - Aquarium Advice
The sharks you bought are probably red tailed sharks which seem to be the most common. They will fit in a 55g tank but will be aggressive towards each other and will chase around their tank mates. I would suggest just keeping 1 if you really like them.
As for figuring out how many fish to keep take a look at www.aqadvisor.com
and it will help guide you.