what time would you suggest getting there? I mean I hve park tickets anyway so I can do a park. Whats the easiest?
Well all of the parks open at 9 AM, but Magic Kingdom has Extra Magic Hour mornings that day...which means the park opens at 8 AM for resort guests. So if you are staying at a WDW resort I would try to go to MK.
I am not sure if the pin store will be open at 8, but at least you will be in the park and can hang out around the closest entrance to the store when they open at 9.
If you are not staying at a WDW resort I would probably avoid MK, and go to either Epcot or the Studios. Both of their pin stores are relatively close to the park entrances...Epcot usually has more pins than Studios, but if you get there when they open neither should be a problem.
Animal Kingdom should also be fine if you get there when they open, but their selection doesn't seem to be as large.
For the Lion King release, Downtown Disney implemented a system where they divided all of the PODMs into categories (such as just Simba, Simba with Nala, Simba with Rafiki, etc.). You were able to look at four PODMs and choose which two you wanted to purchase from those four. At the parks, they did not use this system and just put them on the shelves where you were free to look at them, although they only put out around 25 at a time and only replenished after all (or most) of those were sold. I am mentioning this because it was rumored that a fight broke out at Epcot on the morning of the Lion King release, so I am not sure if they will start implementing the DTD system at the parks for future releases or not.
I do not expect the Mulan release to be as crazy as the Lion King release, because the movie is not as popular and the crowds will be much lighter in September...but I would still get there when the park opens just to be on the safe side. And if you want to park hop and get multiple pins, I would definitely suggest getting there at opening.