In one of my 10g tanks I have a male Betta with 6 Harlies, and some snails. You could do Harlies, neons, or cardinal tetras in there with the Betta. But add slowly and watch closley as they may nip at the Betta, or the Betta may be aggresive. Actually, he will be aggresive, but should calm down soon. If he doesn't then he will need to be rehomed.