I don't think that's an idiot question more like a good one. Although the volume of water would more than likely be sufficent to raise a Rhom the footprint wouldn't be.
As Rhoms get older they become much more powerful swimmers and would appreciate 6-8 ft in length so as to not blast the glass w/ their noses. If I'm not mistaken Rhoms get taller than Oscars, almost as tall as they are long. The Oscars build is more streamline and also much more arrticulate, they can turn around alot easier in tighter spaces than Piranha can. I wouldn't recommend either fish in 75gal. as adults but it's a good growout tank for both.