different sizes for joists on a deck
It is ok to use all 2x12's of course, and ok to use 2x10 joists on a 2x12 band.

Other ideas ...

1) use lower grade 2x12s in place of the 2x10s

2) use all 2x10s placing them 16" on center for the short spans and 12"oc for the long spans

3) look into cantilevering the joists on the long span, which may allow using 2x10s @ 16"oc (instead of uncantilevered 2x12 @ 16"oc).