Im planning on getting some neon tetras and some otos for my 20 gallon which my betta Varro will also be in, just 1 question, how many fish should i get/ max amount, and how would i go about feeding them? what food?
For your stocking questions, you should use this calculator: http://aqadvisor.com/ it will take into account the dimensions of the tank as well as your specific filter.
Has the tank undergone the nitrogen cycle? If not, you should definitely do a fishless cycle before getting any fish. This takes less effort than the fish-in method, doesn't risk the health of your fish, and allows you to add a lot of fish at one time which is important when you want schooling fish like neon tetras.
I also suggest reconsidering otoclinus catfish--because all the otos available are wild-caught, they're very sensitive and do not take well to prepared foods. They eat almost exclusively algae, and of that algae only a few very specific types. For this reason, they're only recommended for older, established tanks, in which you can cultivate enough algae to sustain them. New setups don't have a reliable food source for them, and most otos have already starved in the pet store.
Unless we knew the brand of your filter, the cartridge in your filter, the exact type and dimensions of your tank, the types of fish you're thinking about getting, how planted your tank is going to be, etc, we cannot tell you an exact amount of fish that you can put in there.
A general rule is one gallon of water per inch of fish. Bettas can average 2.5 inches. Neon tetras about an inch. So you'll have to do a little bit of math yourself, and if you think you got the numbers you need, plug them into that calculator and make sure you're not overstocking.
Ok cool thanks! im probably just going to put about 10-12 neon tetras and my betta Varro so they can have some more room, Thanks :D does the calc do food amounts also? because i sont have the tank yet, i was gonna buy it later, then agter i cycle it i was gonna get the fish. so, average amount of food and what kind?
I would be careful with neon tetras in vast amounts, because even in a 20 gallon your betta wouldn't be able to get away from that many, and last I checked neon tetras are notorious fin nippers, they'll tear him to shreds. Tetras can just live off of tropical flakes but not too much or the food will spoil your tank. When feeding the tetras I recommend scooping up Mr. Betta in one of those breeder's traps so he can't get the flakes and you can feed him his good protein-enriched betta pellets like OmegaOne Betta Buffet Pellets, New Life Spectrum, Atison's Betta Food, etc. :)
I prefer New Life Spectrum small fish formula pellets for my community, HBH is also a good brand. It's a good idea to supplement their diets with frozen foods occasionally, I use hikari's frozen brine shrimp, blood worms, and daphnia. For the betta you should get a larger pellet, such as OmegaOne Betta Buffet Pellets, Atison's Betta Pellets, and Ken's Betta Crumbles. Once you get your fish, you'll get the hang of feeding them pretty quickly.
Make sure that if you're getting that many tetras that you get a 20 gallon long instead of a tall or hex. They will appreciate the swimming space. With 10-12 tetras and the betta, your tank would only be about 50% stocked. If you wanted you could get a school of pygmy cories as well, 6 of them would bring you up to about 60% stocked. The tetras shouldn't nip if they're in a big school like that--where people have problems is when they only put in a few and then keep them with a long-finned betta--that's just asking for trouble.