Wow you really need to back down on fish #s, what you're thinking right now would have this tank overstocked 2-3x over.
First of all, a 20H tank has the same footprint as a 10gal tank. So Cories really don't have much room. Adult C. schwartzi are close to 3", so I wouldn't keep any in a tank this size at all. If you want Cories, I'd go with much smaller species, as they should be kept in schools. Pygmy Cories would be ideal.
Otos also are best kept in small schools. You could easily do 3-5 in this tank, as long as you're careful to keep them fed with fresh veggies.
www.Otocinclus.com.
I wouldn't keep tiger barbs in a tank this small, either. They need swimming room and can definitely get aggressive when kept in too crowded a space. I kept 8 in my 46gal, and IMO that's the minimum size you'd need for a school of adults. Same issue with bettas. Though you may be able to keep 4 or so in this tank... possibly, depending on what else you have with them (you wouldn't have room for much more). Personally, I never had much luck with betta sororities- I always had aggression and jumpers. It's also not a good idea to mix bettas with gouramis.
I think you'll have more success if you go with dwarf/nano fish for this tank; then you can keep larger numbers. I think a school of dwarf cories and dwarf rasboras or small tetras would be a much more appropriate choice and give you better chances for long term success.
