Probably a good plan.
I started with a 50mm, which I really loved because like landis said, it forced me to look at something and very carefully compose it to get only what I really need to get in the frame. At first I was kicking myself for getting it, and always wanting something wider, but then got used to it. I think I improved my photography much more quickly with the 50mm than I would have with something wider.
So, applied to the fisheye, not only will you have distortion that usually looks bad unless you're doing followcams and handrails (and I honestly prefer non-fisheye shots in almost every case anyways, personal preference), but you'll often end up with too much in the frame. Also, if you want to take photos sometimes instead of video, a fisheye becomes even more specific and not useful for most situations. There are a lot of more useful lenses I'd get before going for a fisheye.
Also, the tamron 17-50 comes in versions with or without vibration compensation. The vc version is more expensive, not as sharp, heavier, and not really useful because for skiing you will want more stabilization than it provides anyways. The non-vc version is probably the most-recommended lens on this site.
Skiing's rad, hating's bad.