You can add lots of toys and treats to keep them amused. Remember at a rescue place each cat has much less space than a room, sometimes a glorified box. It would be really sad if you have to send them back, but I will say I've adopted a lot of adult cats and it's never happened to me. There's a chance, but I think it's fairly low.
Have you considered fostering for a rescue? That way you can save a cat and you can get a better idea how your cat will react.
Why are you doing 7 hours of homework as an adult?