One of the milder peels we offer in combination with medical microdermabrasion is Agera, which I think would be a good peel for a beauty therapist to offer. It really depends upon the indications.
I would also highly recommend Wigmore Medical for training. You will be training with providers who are medically experienced with a sound knowledge base. (The are also a pharmacy for medical aesthetic practitioners/prescribers). They offer a wide range of peels - some prescription only, some suitable for beauty professionals. They do not have any vested interest in any specific system, and will not pressure you into choosing one peel over another. Personally I think if you are going to train in peels, you need to be looking at a company who have a medical background/ethos.