the zenmuse that is sold by itself, not with the phantom 2, is really meant to work on the phantom 1/450/550 but can work on a phantom 2. I have a stand alone zenmuse on my phantom 2 right now. The zenmuse that is sold with the phantom 2 only works with the phantom 2. The reason is because it doesn't have a GCU (the phantom 2 has it built in) and the hole pattern doesn't fit anything else.
as far as the little white plastic gopro mount, there is nothing special there. You can make new holes for it. Personally I would use an actual gopro double stick tape mount instead. I think it will be strong than the plastic one with holes in it.
As far as aftermarket gimbals working on a P2, most of them do not line up with the P2. A lot of people are making mounting plates for them but I haven't seen any yet. Although if you buy a P2 and want a gimbal, get the P2 with zenmuse package. Its plug and play and for under $200 more, you can get a better 2 axis gimbal than that. If you want a 3 axis gimbal, you will need a new mount anyways because they are taller and so far none fit under the P2.