If you want a fish with somewhat similar appearance to a redtail shark that is more compatible with goldfish, consider golden shiners. Adults are a brassy color, similar to a common goldfish but shinier, and they have red fins. They are similar in size, temperament, and diet to comets, and are readily available at bait shops (of course, you must quarantine any bait shop purchases). They're also easy to collect from the wild in much of eastern and central North America.
bala sharks need warmer water and the IR sharks will eat the goldfish eventually, not to mention they're nearly impossible to house.
my favorite goldfish tankmates are white cloud minnows.