A group of raimbows would bw ideal. Bosemani's, turquois, Axelrodi's , praecox would all be great = or some of each.
Or, how about a bunch of cherry barbs or rosy barbs?
munkee, an Iridescent Shark would not be stuffed in that tank. My cousin-in-law had an Iridescent Shark in a 40 gallon tank for seven years. It was about a foot long.