Which fish combination is the best to have?

You can actually add much more of a varety then that, the only problem with beta's is their fins. they're only aggressive to other beta's, male or female, but the other fish tear up their fins. Molly's tend to pick on anything else in the tank, and sometimes placo's (sucker fish) try to eat them. What I would worry about more is the size the fish get in the tank. Most fish get along just fine with those frogs, they usually ignore them. I would get an angel fish and a couple different types of little fish, like neons and tetras, or a couple of guaramies, like kissing or blue, and a few little fish. What ever you decide, I'm sure it will look nice.