Really I don't know that any should be housed together, they are all aggressive fish, I know so far you have had no problems but I would imagine in the future you will.
Size wise the oscar requires a 55 itself, the pacu I believe requires like 180g, I don't know which "shark" you have but most get really big requiring like 125-300g depending on what you have. These are just to start with...They have a species profile here maybe go there and see if you can find your fish. The cichlid would be fine in your 55 along with some other cichlids or tank mates, or the oscar...well he will in the end eat any tankmate that is smaller than him as far as I know. The pleco I don't know what you have some can get extreamly large.
I've learned all this the hard way too...not the same fish as you but ended up with alot of fish that would grow WAY to large for thier tanks, and the whole aquairum salt thing.