Well, i can't help you on the technicalities, as i'm no good at that stuff. (my hubby handles all that)
But i can tell you, i like to mix em up. usually a comet, a common, a fan tail, an oranda and a lionhead, a mixture of colors and shape and sizes.
you can look at my gold fish album on my profile for my goldfish babies.
see attached link for types:
http://www.kokosgoldfish.com/ftypes.html
i did have a couple get swimbladder from floating flakes, so now i soak their flake food before putting it in the thank, so it sinks and that seems to have helped. i tried sinking pellets, but they didn't care for that, they like the flake the best.
good luck!!!