You may use a 7.62 suppressor on a 5.56... most of the testing I've read or heard about, where a dB meter was used, indicate that the difference between a dedicated 5.56 and a 7.62 can on a 5.56 is 3-4 dB... which is about the same as the high to low spread that many suppressor makers advertise.
I've got a buddy (yeah, I know, a "buddy"
) that uses a 7.62 AAC Cyclone on both his FN SCAR 16 and 17... absolutely loves it.
Cheers,
C