In my experience, the heads come off pretty easy, but if your's is on tight, I'd suggest putting her in the oven for 30 seconds to a minute at the lowest setting (somewhere around 250 degrees).
as far as hanks of hair, I'm not sure what to tell you. I'd assume you'd need at least 5 or 6 of the MLP hanks, it would also depend on how long you want the hair, if you want it SLH you may consider getting the hanks in baby doll size rather than MLP-sized quantities