1.) I think if you're willing to spend your money and time on a larger tank, your mom should respect that. Sure, you had to learn your lesson the hard way, but at least you are willing to recognize your mistakes and correct them. That puts you way ahead of most people. :) I have trouble communicating with my parents, so I often type/write stuff up when I have something important to say. Maybe doing that would help you too. I would go through and answer any potential questions she may have. For example, if she often brings up the amount you spent on the initial tank, explain to her that you realize in hindsight that it wasn't the best route to take, but that you have at least learned from it. I would also explain to her your views on fish themselves. You can't force her to believe that fish have at least some basic level of feeling and thought (even though they do, haha), but you can try to lay things out for her and hope that she will respect your feelings.
2. Teehee, stocking, my favorite part. Well, first, if you got the 20 gallon, I would actually get 4 more otos and 4 more cories. If possible, I would also rehome one of the dwarf gouramis. Most chain pet stores only sell the males because of their coloring, but two males will probably fight when they mature.
HOWEVER, if you got the 50+ tank, you could actually keep both dwarf gouramis if you got three more. They do okay kept together, but only in larger groups (at least 5). Unfortunately, angelfish are not compatible with your current fish, but you will probably like the long fin black skirt tetras. This is just a proposed stocking, but here's an idea of what you could end up with:
-6 sterbai cory (make sure they are all the same kind)
-5 dwarf gouramis (can do a combo of powder blue, fire red, and regular)
-6 long fin black skirt tetras (can do a combo of the black and gold skirts)
-6 harlequin rasboras
All of these fish love plant cover, especially the dwarf gouramis, so make sure you have plenty of densely planted areas for them, but leave enough open space in the front for the more active fish to be able to dart around when they want. Live plants are always beneficial. Mondo grass, anacharis, and sword plants are hardy and are good beginner plants. Your otos will especially love live plants, and they also benefit from live driftwood as well.
By the way, if you want to save a ton of money, check out craigslist and other local classifieds for aquariums. You could easily get a complete 55 setup for what a new 20 would cost.
Alrighty, I hope this helps. If nothing else, I had some fun thinking up a potential stocking. If there were other fish you had in mind, let us know. Good luck, I hope your mom at least let's you upgrade to the 20! :)
Edit: Can't believe I forgot to mention the 6 gallon! It doesn't have to go to waste at all. It would make a perfect home for a betta and some shrimp or a snail. Bettas are one of my favorite fish. :D