I would recommend ditch the other fish (i.e. find them another home) and keep the Oscar. The 55 you have is ok for the Oscar but anything else will cause you alot of grief down the line. You would be changing water every 3 days or so.
Also, you mentioned
"silver dollars might not last to long when the oscar grows, wait and see.."
Don't let it get to the point that that situation is even an issue. Worrying about whether or not several species of fish can interact together in a smaller tank is NOT where you want to be, and will only cause both you, and the fish....undue stress. You should enjoy having them and not have to worry, IMO. Some people use a 'trial and error' method to see if fish can interact, however I get rather attached to my fish and would HATE seeing them be killed/die due to my own negligence, either in tank size, or incompatibility.
Remember the point of keeping fish is to enjoy them, and enjoy caring for them, and seeing them thrive in your
own constructed ecosystem. How would we feel if we knew a tank couldn't support all the fish we have or that we had to worry about one fish eating another day in and day out? I would surmise that is NOT a situation we would like, nor would it be one our fish would care for very much.