I'm not familiar with turtles but I can talk about the Red Tail Shark and the Betta.
Red Tail sharks are very active territorial fish and in order for them to be happy they require a tank at least 36" long.
Betta's are very poor community fish since they're long fins slow them down (turtle chow) and other fish really like to pick on them.
Now I've never had turtles but from what I understand they are very messy so over filtration is a must. Usually the only fish you'll see in with turtles are feeder fish.
EDIT: I did a bit of Googleing and found
this site. Id suggest as a stocking senario that you over filter as much as you can. Look for fast agile fish. And try to give the turtle at least 10 gals of the total tank volume to himself... though ideally should probably count a bit higher around 20 due to waste and the lowered water level not holding the full tank vol.
Hope that helps.