Like I said earlier, I'm setting the tank up for my moms boss, and she said she didn't want any more bettas, as she already has one male, and she finds them boring..Corydoras are schooling fish, and unless you went with Pygmy Corydoras, 7.5 gallons is too small IMO for a decent sized group.
How about 3 female bettas?
I'll definatelt ask if the betta will be going in or not..Bettas and gouramis are a big no-no. They'll kill each other very quickly. If the betta is definitely going in there you'll be very limited, depending on the temperament. I had a betta that tried to kill my otos every chance he got. Luckily he never succeeded. Shrimp might work with a betta. Tylomelania snails would be a big hit. They large, weird, and beautiful. The betta may try to nip their antennae.