Glofish are pretty, and bright! But..... thats bad to mix with betta. My brother tried to introduce some glofish into his betta, but they were too flashy and the betta attacked the poor lil guys. Im pretty sure they are just danios, correct me if im wrong. But regular danios or zebra danios are better cuz they are not so flashy, so less attracting for the bettas to attack. Any who..... glofish are mad expensive any way... i see them from 4-6 bux.... kinds ridiculous i think, just cuz they are bright and pretty haha. Good luck if you try!
Long fins are a rule of thumb are usually no go, but if you would like to you could try! You never know. Also like mentioned above Rosy Red minnows are fun to watch. I like mine. I would be careful with the adding so many fish at once, you could disrupt the cycle. id add the corys then 2 weeks later add the danios.
My Betta, Jasper, has a Blue Mystery Snail buddy "Gary". Other than trying to taste-test him at first introduction, he doesn't mind his new tank mate and even swims over to check up on Gary often to make sure he's still around.
Once I get a 10g set up, I will probably add a 2nd one. They're neat little guys, I think.