What body are you using? You didn't mention video in your OP, so that changes things up a little bit, don't ya think? if using to film, not all lenses will auto focus with the video you are shooting. You may need something more dynamic. My experience with it is limited, and wave/surf based, but even with skating vids, most seem to be filmed with something along the lines of an 11-16, something really wide, where you can just mount it and ride along side. Primes, for something like shooting skating vids, perhaps NOT the bet choice. With no body info, it's hard to recommend something that will have the ability to stay in focus while you pan, move, skate along. . .Also, as you stated, gonna be hard to zoom while you are skating.
When it comes to name brand v. off name brand, you'd be surprised. . .some lenses I have gotten that are NOT Nikor are stunning, while others, well, lets just say, I got rid of 'em. I have used the Rokinon 85mm (your aforementioned lens) and to be honest, I was pretty pleased with it.
If using a prime to film, you need to jut find/stay on a line that is parallel to the skater and keep everything within frame I imagine. If YOU are on a board, trying to focus on a skaters line, and all the other sh*t that's involved, it just seems like broken gear waiting to happen. If the skaters line is to where you can be in the back of a truck, and shoot/film, then you stand a much better chance of being able to zoom in/out, and do what you need to do, but even then, it takes a LOTTTTTTTTTTTTTT of work manually, lol.
Jarred does mostly automotive work. He started with 99% automotive work, has done the 757 "car scene" for the past 6 years or so, has gone to doing a LOT of video, all using Nikon Gear, IIRC he used to do some skating stuf as he was learning it all but, check out his stuff here:
http://vimeo.com/jaredhouston
It's all the same principals. He's a really great dude and gets better and better every time he posts , it's an automotive board that I moderate, but we have a lot of photo folks.