Personally I'd set him up in a 5-10g by himself, then design a peaceful community for the 20g. For a few reasons;
1. The only way to find out whether you have a peaceful betta is by testing it out with other fish. You're gonna want a spare tank handy in case he starts on a killing spree overnight and you have to rescue any survivors.
2. Have seen way too many posts on this forum from people asking what to do because their "normally peaceful, I swear" betta has begun murdering tankmates. Just do a search for "betta killed..." and you can read the stories for yourself.
3. Male bettas don't live in communities in the wild. They search for a territory, find one, and ferociously defend it, only allowing a passing female to stay long enough to lay eggs - if he likes her - before even she gets chased off. Betta breeders emphasise how important it is to monitor when the pair have finished laying and remove the female quickly, because it's so common that the male will chase her away after mating. But in a tank with no escape, he will kill her. So it isn't how they live in the wild. I personally believe we should try to recreate wild conditions as much as possible, so social fish need to live in groups of the same species, while solitary fish should be solitary.
4. When questioning something to do with aquarium keeping, I like to ask the question: "who benefits from this?" I you want a larger tank, and both you and the fish benefit, then great. Lots of ugly fake decor instead of live plants/natural hardscape? For the owners' sake, not the fishes.
There's no benefit to the betta in having tankmates, and there's no benefit to the tankmates by having a betta around. So if you chose to take the risk, then have to acknowledge that there is no benefit to the fish, and it's risking it for your own benefit.