I guess, the question sounds a little bit more complicated ... To clarify, I'll start:

I use my 4x5 to shoot 2x5's. I don't have any other adapter like 6x6 back or 6x9 back.
My 8x10 Dorf is used for 4x10 and 5x8 as well.
My 8x20 is used to shoot 7x17.

How about you?