There are some interesting ideas on Betta tankmates in a 20gal tank.
Please read this interesting pinned post at ultimatebettas.com
http://www.ultimatebettas.com/index.php?showtopic=14389
I can tell you that if you do a good
aquascaping work the stress due to tank size is likely to disappear.
There are reports of huge intra-species temperamental variations among Bettas. Not all male bettas are bull-fighters.
I can tell you that I rescued a fighting-breeded specimen from a
LFS -kept out of sight from regular clients; somehow I found this guy along with a few dozen more in tiny glass jars.
I took him home, and he shares his 5.5gal tank with three Ghost Shrimps and a Wild-caught Nerite Snail.
I have kept Neon juveniles in there with him with no problems.
My dearest Edda, who passed away seven weeks ago, shared her tank with a school of Zebra Danios and even Fancy Guppies (I don't recommend Fancy Guppies as tank-mates for Bettas: when I tried to relocate them in much bigger tank, she turned really mean with the male Fancy Guppy).
Take your time, your idea might work well. Just have a plan B in case it doesn't. Situations like this one are a hit-or-miss.
Pepe
Santo Domingo