I say the cardinals as well. I would recommend Rummynose Tetras though hands down. I have a 37 gallon with 6 cardinals, 7 rummy's, 8 white cloud mountain minnows, 2 bolivian rams, a clown pleco, a betta, and lots of ghost shrimp. As I get other tanks together, I will be moving the Cardinals to a tank where I can have a much larger school and get more rummys. Both the cardinals and rummys stay in the middle to the bottom of the tank. The rams never mess with them and the betta stays towards the top and I am lucky that he is not really aggressive to any of his tank mates.
I ended up getting a betta after all, just because I saw one with colors I couldn't resist. The store I found him in had him in a 10 gal with tiny fish and he just ignored them, so I'm not worried about him with Cardinals. I did more research on Gold Barbs and I'm pretty tempted. Them and the Cardinals together would look fantastic
My betta stays towards the bottom, wish he would go towards the top!