Coskie, they won't overshadow the Synergy speakers as you do have control over the amount of volume(level) that will be applied to them, the centre is sensitive so that you can turn it down withoput sacraficing quality in your sound. As far as the rears go, same thing applies and they aren't that much different than your mains, depends on how much power you will be applying to these per channel.
If its not very high, staying with the Synergy set will make you happy.