Good advice above.

If it's working don't break it.

Unless you have a reason. If you want to try him on rats then you can - but you need to make sure the feeders are well watered and fed beforehand and aren't distressed - it minimizes the chance of things going wrong.

You will still need to supervise and be ready to step in if there is a problem though.

Bear in mind if he is used to the scent of mice he may not immediately see a rat as prey.