Well, you can't do the gourami (the bettas will fight with it). For corys, they prefer 72-79 degrees, and bettas like 76-82 degrees. Try to find a healthy balance. I would do around 78 degrees, but this could be debatable.
30 gallons is pretty big so you should be able to keep a good number of khulis too. They like having groups even though they're not schooling fish per se, and you'll see em more often with groups of 6 or more.