i would not encourage on rats until it has been started on the bunnies will only promote a hard mouthed dog imo,its just a pup mate it will soon get the hang of picking them up im sure,be patient,bit young for my liking to be running quarry yet anyway but that's another story.
once established on the bunnies then you can turn it on other quarry they aint stupid and learn easily the difference between something that bites back and something that does not, don't want a dog killing bunnies anyway if you ask me,retrieve live to hand is something i all ways try and strive toward but each to their