Welcome to the mad world of Poser. 

Regarding compatibility between models, hair can be repositioned, scaled and parented to an incompatible figure but it could be time consuming depending the complexity of the piece.  Netherworks does have a hair conversion kit for the new generation of DAZ models.  You might want to take a look at the specs to see if it fills your needs and whether you will get plenty of mileage from such an app.

Using other individual's artwork for commercial venture is rather risky unless you have a written agreement with the vendor or it is specifically stated in the product readme.  It is always prudent to double check before committing yourself.  Even tattoo art commonly seen on websites are protected under copyright laws and unauthorized commercial use in other media will only get you into legal problems.  Better to create your own, copyright it as your own, and you will be free of any liegal obligation.

Again, welcome to the club.