Again, coming from another umpires viewpoint, I see very little that is gained from 13-14's playing on a 60/90.
There is plenty of time to get "used" to the big field.....
I would recommend that you pick one and stay with it....the struggling to adapt between the 2 choices can bring frustration to all involved....
I believe the 54/80 is the best transition from the small diamond to the big one......