I just got my ezpass for my motorcycle in the mail today. It is a special tag for just motorcycles. All you have to do is call ezpass customer service and they will send you one. You could use a regular car tag, if you wanted, but you would get charged the higher rate for a car. I think the rate for motorcycles is 50% of the car rate, but am not totally sure.
As far as mounting it goes, I think it would fit naturally behind the windscreen and it is safe from coming loose and falling off the bike. One thing though, it is branded with a large "M" and the thing, IMO, looks downright dorky when viewing the bike head on. I want to see if I can find a way to mount it on either of the inner fairings.. there is a flat spot near toward the rear near where it meets the tank. If you have a tank bag, those sensors should be able to read it but I suppose the first time you use it, approach with caution.
One more thing, make sure you register your bike's plate # on the website because if it fails to read your tag and your plate is not registered you get hit with a steep fine.