Regarding the bettas, I've kept a single male with a group of females before, in a community tank. The females caused each other more problems than the male did, but they didn't do any lasting damage.
A 20 long....hmmm......the 1st thing that pops in my head is pair of Kribs with maybe some Leopard or Zebra Danios up top...or maybe just a school of glass cats or a single elephant nose.
I was just reading up on Lake Tanganyika shell dwellers. They seem like really interesting fish to keep, and you wont need any fussy plants. A 20 long would be great for a small colony of them.
You could breed a pair of kribensis in there. Very cool fish, and breed readily with a cave structure or something. You could also add other fish, as well. Kribs aren't really aggressive cichlids.
Are you SURE you don't want to try live plants? It's addicting ... and your fish would love it.
Maybe your mom would consider getting you some lights and/or a hood for that thing for Christmas (since she's no longer going to get the 10-gal). And a heater. And a filter ...
And if it's been frozen, it would be a good idea to just fill it with water once you get it clean to see if it will hold up for a few days without leaking.