It is a very hard call; I too have worked at a pet store and I have also rescued poor little ones from pet stores. Since the pet store is already purchasing from supply agents/mills and most likely won't stop I would say the best thing to do for the hedgehogs is to get in there and give them the best quality of life you can while you are there. In addition to this, you will have a chance to earn the trust of your co-workers and your customers; therefore, you will be in a great position to educate fellow employees and customers on proper hedgehog care and why purchasing from a huge supply company really is not in the best interest of the hedgie-- they probably won't listen and will still buy them as long as there is a demand for them, but being heard is what is important. All in all, I think being there and having the opportunity to make a bad situation the best you can is fantastic. There are a lot of people out there that love hedgehogs and think "they are so cute", but have absolutely no idea how to care for one and this is your chance to make a difference
Best of luck!