Tinfoil barbs? A school of them. If when you upgrade which you will have to for the ray maybe get a small 5-6" arrowana. A fish at the top and a fish are the bottom look really nice, classic combo.
yeah, actually here's what you avoid keeping with rays : bichirs, plecos, gars, larger catfish species, snakehead, anything too aggressive or too active. Active fish tend to stress younger rays.