My worry with the ultra sonic correction collars would be that the noise might really bother your rats. I know there are "pest control' devices that work in a similar way, where they emit a high frequency noise to keep the rats away.
There are a couple of things you can do. First, I'd figure out why he's barking at the rats... is he curious? is he scared? does he want to play with the rats?
if he is scared ( scared barking is usually from a distance, and the body will be very tense, tail straight out) then teach him the "touch" command. First start with your hand, teach him to touch by saying "touch" gently touching his nose, and giving him a treat. Then after a few times, move your hand a few inches from his nose, say "touch" and wait, he will be confused at first, but he will want the treat that usually accompanies that word, so he will try to figure it out. If he just stares at you, move your hand closer, but don't touch him yet, wait for him to touch you. If he doesn't get it at that point, go back a step and start over. Once hes masterd touch, have him touch the rat cage (I wouldn't do this with the rats, because he may not react well) Keep him leashed through this process, so you can pull him away quickly if he reacts badly. Most likely he will be focused on the treats. This time say "touch" and point at the cage with the same hand you had him touch before, and reward him for this. This should help him become more comfortable with the rats, and he will learn that they are nothing to be concerned over.
If like my roomies dog he is curious (body more relaxed, ears forward, body ingeneral more forward) then you can do something as simple as taking an empty aluminum can, and put a few pennies in it. When he barks shake the can. If this method is going to work, you will know right away. He will react to the noise, probably by looking at you with a quizical look... like WTF was that?! at that point, because he's stopped barking, say "quiet" and then give him a treat and praise him for being quiet. If he starts to bark again repeat the process. Try not to let him see the can, because you want to distance your self from it, so he wont bark when your not home, because he's worried about the big noise coming back.
If that doesn't work, and he is interested in the rats, you can remove him when he starts to bark. Find a safe place to stash him (not his crate, because his crate should never be a punishment) but some where he cannot see you, or your rats. When he barks, simply grab him, and put him in the other room. Leave him there for a few minutes, and then let him back into the room with the rats. If he barks again repeat the process. If he wants to be near the rats, to check them out and watch them, then he will learn that he needs to be quiet to be out there. When you let him out and he is being quiet, tell him what a good boy he his, and give lots of love and treats. If then he starts barking again, repeat... this takes a little while, but its very effective.
Hope that helps
