You're not allowed to use a dressage in E/P or Equ. Because they need to see your leg postioning and if it's not a dressage show you will probably not win. I don't know what level this show is but mostly all of them will not allow a dressage saddle.
If your just going to a local show to try it out then a dressage saddle is fine. If you decide you are going to get seriose about showng it then you'll want to start looking at some saddles. Especially if you start going to rated shows, they will call you in