Hoosier Rains

I just bought a set of used presumably (one session old) Hoosiers rains the sizes are 22.5 x 75. - 13 rears and 20.5 x 6.5 - 13 fronts.



Ive been told to go with 21s on the fronts but this guy only had 20.5s. how much of a difference will this make if any? Also can I mount these on the same wheels as my dry tires?



Thanks!