They are mostly peaceful depending on the species. I would suggest some red tails. A full grown one of them swimming around would woo alot of guests lol.
large barbs are pretty nice, but not usually not colorful as you want. still very pretty. i like java barbs
perhaps angelfish.
goldfish fit what your looking for perfectly i think, i thoroughly enjoy the goldfish i have. i dont know i would devote a whole 500 gallon tank to them tho. but then again goldfish would be the cheapest to keep probably since you wont need to heat the water.