Hey! I know the answer to some of these!!!
Why is it that Bettas CAN live in a smaller enviroment than GF, like a 2.5 gal tank?
Betta's produce significantly less waste than GF and they don't get nearly as big as goldfish
Also, why is it that bettas can live w/out a filter as long as you change the water like you should?
Again they don't make as much waste.
Why can't bettas be put together, why do they fight?
Very terrtorial because because of the tight conditions their used to in the wild.
LASTLY (LOL), how can you tell the sex of a betta?
Male Betta has more fins and is usually more colorful

Female is smaller, smaller fins, and most of the time is less colorful

I think most of that's right according toi what I've read lately but I'm sure some one will correct me if I'm wrong! LOL