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.

I can't tell you what to do, but maybe my experience will help you. My wife and I got two cats 9 years ago, a third two years ago and last year, unfortunately, one of our cats died, so after a while we filled up to three again because there was stress between the other two. I would say that what we see as "aggressive" is actually more defensive behavior, every time we brought in new cats there was a lot of hissing and growling. We always kept new cats in a separate room for 7 days, with a close-meshed door net attached to the door frame so that the cats could sniff each other but not touch. This has always worked really well so far and in the end they are always inseparable. I would try it, friends of mine had different experiences and had to give up a cat. But that's not the end of the world either, in the end the animals should feel comfortable and if it doesn't work out, you can find a better home for the new cat. Yes it would be stress for the new cat if it doesn't work out, but in the other hand there is the possibility that it works out and it is less stressful for the animal than when it stays in the shelter.

Additionally in between we found a baby stray cat and brought her up (took about 3 weeks of nursing) and our cats were much more open than with new adult cats. Maybe this baby bonus could work with your cat as well if you want to invest the time, it was a great experience. Especially because the cat would have died if we left it where we found it.