To be honest you can only have one type of schooling fish in a 10g. If you're talking about catfish I'm guessing you're talking about Cories?
They're wonderful bottom dwellers and they're peaceful community fish. First off those have Cories though you would need a sand substrate, if you don't have sand as your substrate, tough luck. If you do you could do, Julii Cories, or Pygmy Cories. I'd vote for Pygmy Cories, because they're cute and small, could fit around 10 of those in your tank, with one Betta. Pygmy Cories
If you don't want Cories, Ember Tetras could work, they're small and lovely. :) Could also do around 10 of those also. These guys prefer lower pH, if they live in alkaline conditions they're colors get kinda pale and washed out.
To QT them, I would QT them in the 10g and you could move your Betta in other tank if you want to. But to be honest, I always take the risk and I don't QT plants/fish when I get them.