Sounds good. Since all the NS speakers are made to go with each other, there should be no problems using the 777s with the 333s as surrounds. No matter if you go for 5.1 or 7.1, I would recommend that you opt for the 333s for your rear for speakers. Then the 777s up front and the 444 as the center. If Yamaha made dipole surrounds, it might be a bit harder to choose. But seeing how they don't 333s all around in the surround channels.
Oh, and no, they shouldn't overpower your other speakers. If they end up being too loud (doubtful), you can always adjust the levels on your receiver.