look at it like comparing 2 engines-- one with 80 hp, and one with 100 hp. both would propel the car, and run at similar rpm's, but the larger motor would have a little bit more 'oomph'. same for speakers.
it is fine to run speakers with a little bit less power-- the power rating is mostly derived from the way that voice coils can get rid of the heat that gets generated-- the better they get rid of the heat, the higher the power rating..
i am not crazy about mixing speakers-- in a 7.1 setup, it can mess up the imaging a little--though, if you can tolerate it then i would say to use them as the center/phone speakers-- they will probably have the most benefit there.