I would borrow a humane trap from the Humane Society. Make sure the kittens are really hungry, and put something that smells great in the back of the trap. Put the trap in a natural hiding place, and try to be hiding near by, so that the kitten doesn't panic. Have the vet's office informed of when you're going to try to trap them ( or one at a time). Have a towel or dark cloth ready to cover the trap when the kitten enters. That will also help him to remain somewhat calm. Then take him to the vet as soon as possible.
Show Rusty the litter box and scratch his little paws back and forth. He will want a place to use.
I would introduce Rusty to your other cats slowly, first keeping him in another room, while exchanging bedding with the other cats--about a week. Then exchange sleeping rooms.
If you can find a way, the second week would be the time to allow them to see the other cats through a small opening in the doorway. You could hang a toy for interactive play also, but wait a few days before they are in the same room together. You will have to use your judgment on the number of days for each step, depending on the cats' reaction.
After a few days, put Rusty in a carrier or play pen and put him in the same room. Watch the reaction, and when you feel confident, a few days later, allow him to meet one of the cats as you supervise. If that goes well, repeat the last two steps for the other cat. Take your time! It's worth it!
I wish you the best. Keep us informed, please.
PS Kittens adjust more quickly than cats.