Well, in a 10, I'd say you could do 2-3, depending on hiding spots and what sex they are. I have one in a 2.5, he loves pest snails, in fact that's the only thing he'll touch besides live blackworms. You could easily do 2 males in a 10, the general rule of thumb is 5 gallons per puffer. You could try and determine sex, but it's extremely hard and I don't think you'll want to stand at the LFS display tank trying to figure out their gender for an hour. Males have small wrinkles behind their eyes, but this only shows when they're older. The other signs of a male is when you can see that their "spots" along the body are in a row, ending at the tail. Females, there is no certain way to tell except if they don't show the signs of a male, and they are generally a little wider. As for Oto's, I'm sure that you could do at least 4-5 in there with the puffers. These are among some of the only fish that won't be killed and ripped apart by these puffers, and everyone thinks it's because of how they barely move and just aren't that noticeable. They love rock work, live plants and a current to play in, and I'm sure you'll be fine with 2-3 puffs in there.
Nils