I found this on a website:
"Tankmates: Best kept with fish of similar size such as tetras, platys, Mollies, Swordtails & Corydoras catfish. Unless you want your guppies to become dinner, do not mix them with Cichlids. Siamese fighters (Betas Splendens) may attack guppies because of their similar coloration & long fins. Avoid mixing with Barbs because they have a tendency to nip at other fish's elaborate fins, damaging them in the process (especially Tiger Barbs). As always, make sure the fish you choose can co-exist in the same water conditions. "
I would go for a few tetras, i like rummy noses or neons maybe