Go with more ottos if you can, particularly if you can site the tank so that it gets some sunlight and you can get some algae growth. Two reasons for this: 1) Ottos are shoaling fish every bit as much as cories. They even behave a bit like cories in a large enough (5 to 7) group. Even breeding is similar. 2) Ottos can be delicate. If you put a dozen in the tank, you can be pretty sure you'll end up with between 6 and 8 unless you are very very lucky.
So, 10 female bettas, 7 cories, 5 to 7 ottos, 1 whole snail (Go with Mystery rather than Apple if you can manage. Mysteries stay under golf ball sized, but apple snails can get apple sized) not 2 unless you want to either breed them or regularly pick the egg mass off the tank walls (it's like a lumpy piece of chalk).
I'd be sure to add some driftwood to the tank - the Bettas, Cories, and Ottos all like slightly acid, lightly tannin stained water, and plant it well. Use Crypts, sags, pennywort, hygros, java moss, Java fern, and/or anubias (those last three need to be attached to rock or driftwood rather than planted). Those all handle low to moderate light well, are easy to grow and plant, and are easy on the wallet. Having a well planted tank will improve your water quality and make your bettas and ottos feel more secure, and therefore more likely to be out where you can see them. Finally, make sure that you hardscape so that there are at least two, preferably more places that your cories can get under hard cover (caves, projecting rocks or wood, flowerpots, pvc pipes) - the security of having cover will make them less likely to hide.