I have an Eclipse System 12 (12 gallons) with male betta, what would be some good additions?

I've been up and running for over 3 weeks now, which is a new record for me. The betta is all alone, and I ume the tank has cycled, or at least enough to add a few fish... I was thinking a trio of platies, one male, two females. A local shop sells one type of fish for 99� each the first hour of business Saturday morning. This week, it is red wag platies. Sometimes it is neon tetras, sometimes black tetras. Generally something that goes for $1.49 each to $1.99 each. Last week it was Giant Danios, which I knew wouldn't work out in this tank. Thanks for your opinions.