I agree with optix.3 oscars and 2 dempseys wont last long in a 55gal. If you are just growing them out put them in the bigger tank they plan on living in later as it will only take them a few months to grow big anyways. PLus 5 big cichlids in a 55gal even if they are juvies will eventually thin the group to what a 55gal can actually hold.
What size tank are they going into later? must be quite big if you have 3 oscar and 2 dempseys.
As for decorations you might have some problems. You will need lots of hiding spots but oscars are diggers and will move around anythign you put in with them. So you wil jsut have to wait and see what works for you.
Good luck