I've heard of bettas with shoaling fish without any problems apart from the shoalers fin nipping the betta tbh... BUT I wouldn't recommend it.
PLEASE sort out your current stocking. +1 on the danios needing a longer tank. They're fast, agile swimmers, who need to be in shoals of 6 or more in a long tank. +1 also for the cories needing 6+, so I'd suggest rehoming that too.
I know it seems horrible people on here telling you to get rid of your pets, but (like you?) we just want what's best for them. I've had my 10 gallon up and running for months, and I did a lot of research into the types of fish I could have. You can ALWAYS see something in my tank, and I have the most wonderfully different coloured guppies
If you like platies and mollies, they're fine in a 10G, but make sure you get your sexing right, otherwise you'll end up with LOADS
I don't know what the aggression is like with mollies and platies, but if you went for all males, I'm not sure if you need 3+ to spread aggression (I know guppies are like that, so I'm kinda tarring all live bearer males with the same brush lol... can anyone with more experience clarify?). but you could have, say, 4 mollies, and 4 platies in a 10g, so long as your tank is cycled.
ok I'm waffling now and probably not making any sense...