How about a pair of angels? You would have a lot of choices for dithers that way- most tetra species work great with angels except for neons. Add a school of corys for your bottom feeders and you're set. If you want something different, order a pair of apistogramma from aquabid.com (I seriously doubt that Petco can get those for you). Apistos are South American dwarf cichlids, and even the largest apistos that I know of stay under 4 inches. Many apisto species are very colorful too. A pair of keyhole cichlids would be nice too, they are very gregarious and interesting to watch. Plus, they stay pretty small; 4.5 inches or less.