is it a high or long tank? if its a tall tank, I agree with midthought...bettas dont do well in taller tanks....IMO it lessons their life as they spend so much time'/energy getting to the top for air......if its a long, you may be able to get away with cories... but as others have suggested, each betta is different and have a back up planwhat can go with 5 cories in a 20 long?
I don't want the betta to be in there if there is potential for fighting.
In a 20g? Others are free to disagree, but I think you'd be *okay*, particularly if you had plants (or chose pygmy cories!). You'd probably have to see how quickly your nitrates rise and how often you'd have to do your water changes. However, you're going to wind up with a lot more guppies if you mix your sexes, and you're going to have a plan for all the babies, which will probably quickly stress the bioload.