Yes you can keep imbellis males together if they're siblings and have spent their whole lives together. I keep 4 imbellis (2 males and 2 females) in a 20 long NPT tank.
I feed them .5mm NLS small fish food pellets and the occasional frozen bbs, bloodworm or daphnia. Their care and behavior are very similar to that of Betta splendens.
Feel free to PM me if you ever have questions. I keep wild smaragdina, albimarginata, ocellata, and patoti too. You can also PM Setsuna but I think he keeps his adult males separate. He does have fry now so he can probably send you a pack of juveniles before he separates them.
I have found the IBC website gives acceptable guidelines, but the http://www.seriouslyfish.com
website has far more information on each species. Another good resource is The Betta Handbook
by Robert John. Goldstein, PhD.
Good starter wild species are imbellis, albimarginata, and channoids. They're all small and tolerate a wide pH range.
It is possible to keep some different betta species together (preferably types that cannot interbreed) but a larger than average tank is recommended.