Don't even think about it. They are different sounding speakers. Get the matching REvel speakers and be done with it. Anyting else is cutting yourself short I think.
As I was reading this I was thinking "Don't even think about it" and scrolled down to see forevers message. If you have Revel Salons and don't get Revel Embraces or use Revel ultima Gems for years you have just wasted a lot of money. The M20s might work fine, but it is important to match specific mfgrs. speaker when doing surround. The only exception is in subs, which is where you can vary. I have Revel Studios, Voice, Embrace in my main system. You could also use the Performa Surrounds as rears and People I have talked to at Revel felt that would be appropriate.