ok my gosh i never said i was an expert i was saying that i dont need to know the size of the fish and the size of the tank ive done my reserch on each fish and i needed to know if they would get along not how big they are.
those people are just trying to help you...most of us don't like seeing a fish in poor conditions or hearing stories about a fish that died in a situation that could've been avoided. if i were you, i wouldn't toss their advice aside like it's nothing.
like the info you shared about "piranhas", you made a broad statement. SOME type of piranhas must be kept solo and would NOT be skittish and hide in the corner, they'd attempt to massacre everything else in the tank. it's safe to assume you're talking about a red belly piranha, most research will tell you they perfer to be in a shoal.
just take advantage of the broad range of experience people have here and conduct some additional research and you'll be fine