the betta isnt to slow to be kept with a RTS; bettas can hold their own with other super fast fish like danios. the RTS probably wont bother the betta acctually, as long as the betta stays around the top of the tank.
Hmm.... kinda varying answers. To fill you guys in I got a 20 long that has no fish, just some small snails, and a lot of plants and driftwood. I want to put a fish in and those are my 2 favorite other than a pleco. I am just tossing ideas around really. I don't want many fish just a few larger ones that will be able to live in a 20 long so by large i don't really mean large? haha make any sense. I have other tanks that I can move them into if they get to large but would rather have one or 2 inhabitants in the 20gallon long just to help the plants out that are not too boring or unattractive. Suggestions would be great.
If you're just looking to provide movement in a planted tank, maybe go with 3 or so giant danios and a handful of cories. The giant danios are always moving (as long as the tank isn't overgrown with plants) and are a somewhat attractive, hardy fish.