As you are stuck with a fish in cycle (me too, though for different reasons), maybe you should just stick with your current stock for the moment? The betta will produce enough waste to feed the beneficial bacteria (bb). If you add more, you run the risk of producing too much ammonia for the bb to process and you will have to do even more frequent water changes (pwc = partial water change). I have a 29 gallon tank with seeded media from a cycled tank and only 3 neon tetras in it and they are still producing enough waste that I have to do a pwc every evening! Once it is cycled, then you can slowly add more fish, if your betta will allow them. You do need a back up plan, as many bettas are not tolerant of tank mates. There's no way to predict if yours will happily ignore everyone, or if he will terrorize the neighborhood like a wee mafioso.
While you wait for the tank to cycle, you can plot what you might want to include for tank mates. And you can maybe acquire some aqua-scaping features like rocks, caves, plants, etc. (beyond what you might already have). One suggestion that I have read is that if you dramatically rearrange the aqua-scape when you add other inhabitants, it can reduce the betta's territoriality a bit. Everyone has to carve out a niche. Didn't work for me - both of my bettas are territorial monsters who won't tolerate any company.
Oh, one option, if your betta isn't happy with company would be to section off the tank into two 5 gallon sections with an opaque divider and put a second betta in the other side. Both have the benefit of good water circulation, etc, without competition for turf. But, I am not sure about the mechanics or the effectiveness/appropriateness of this. Perhaps someone with broader betta experience than me might comment?
Happy plotting!