Well, I think yoiu are worrying a lot too early. You don't have the job yet. Do you think you WILL get the job? Around here, if there is a working student job with a USDF silver medalist instructor, there will be several, if not many, students vying for the position. The instructor will want the one with the most "heart" in the business, I should think. He/she will not chose someone who is only interested partway, unless that is all that is available. I know this sounds harsh, but think of it from her perspective. If she is going to give you her time, then she wants someone who really WANTS to learn dressage.
If there is not competition for the job, and you are accepted based on your good work ethic, then awesome ! I doubt your horse will suffer from the variety of experiences. However, to get good at dressage takes years and years of real committment, so if you want to go to higher levels, then you may have to leave go of barrels and put your ALL into dressage. If you give it less, you get less out of it. That's just the way it is.
I used to think I couldn't do Dressage because I was Western so I thought hey why not reining because that was, as some people call it, Western dressage. But now I know different. I'm going to do it. Also what I would learn during the position is stable/barn managment, how to train dressage, how to deal with importing/exporting horses(as that's what they do as well", and so much more.
Yeah I don't know why I hesitated at all! I'm jumping on this one!