No, not unreasonable. We had an old Charmac GN stock trailer that we put a wall in just behind the escape door to make a tack room. Basicaly the divider/cut gate could be used as a stud wall. If I had a strange horse not used to be hauled with mine they get put on the back step, not in the middle of everyone else.
As far as 6.5' tall trailer, I have never had an issue and I have hauled some big draft cross ranch ponies. The only time the height of the trailer has been an issue I have noticed is when horses are backed out to un load rather than turned around and walked out. If the horse is hesitant to step down when backed out they usually raise their head up to try to see behind and down causing them to bang their head on the the cross member. And after they bqng their heads the more nervous they get about unloading by backing out. Usually when stepping out forwards they will lower their head to look where to step down to. However I never let them jump out, they are to step down. I have been told that if they get in the habit of jumping out and slip they can stifle themselves.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.