Lots to unpack here...
First, not all "peppermints" are great at eating Aips. The ones from the Florida Keys (L. Wurdemanni) are the best, by far. It is hard to know that you got this kind unless you order direct from a Keys Diver - ReefTopia.com is a good one that I use a lot. Most LFS and other places have no idea where their supplier/wholesaler got them.
Second, they will not often go after the huge ones since they can get eaten by them. You still need to kill these back by hand with some kalkwasser or your tool of choice. They will eat the smaller aips or the ones in the rocks first and not venture out.
If you get small ones from the supplier, then raise them up in your fuge for a month, or so. They will readily eat flake or pellets and grow fast. They need iodine to molt, so make sure that you change your water. They live about three years, so you will need to replenish your supply every once in a while.