Personally, I'm not a fan of platies or guppies in less than 15 gallons due to their activity levels and adult size. I'm not saying they can't ever work in ten gallons, but they definitely aren't my first choice.
Definitely return the danios - quite apart from space considerations, they also do better in cooler water than bettas, can stress a betta out with their crazy activity, and need schools of six.
I second ember tetras or Endlers Livebearers. If you switch to sand, pygmy or panda cories are also good. :)