I hope all goes well with the new tank!
Amount of gravel is really up to personal preference, but here's a nifty calculator to help you help yourself:
www.aqua-calc.com/calculate/gravel-rectangular-solid
Filtration sounds good. As for the heater, a useful rule of thumb is 5 watts per gallon. If you live in a very moderate climate, less might be fine, if it's very cold, (or the tank will be in a very cold space) more might be needed.
1 oscar and 1 severum/firemouth will be just fine, depending on the individual fish. Some oscars don't take to tankmates, no matter what you do. Some firemouths can be just as bad.
Post pictures if/when things are settled!