I have Mythos 1 fronts, and a Mythos center 8 in my 7.1 system.
For my side surrounds, I selected the Def Tech UIW RSSII's because I wanted bipoles for my sides (and needed ceiling mounted speakers there.) For my rears, I have a pair of 8's to give a more directional feel in the rear most surrounds. It follows one of the recommended Dolby Digital configurations on their website.
Both pairs blend very well with each other and the front three speakers and create a cohesive sound filed because all of the drivers and tweeters are virtually the same. (The Joker shooting the paddy wagon effect just runs around the room in The Dark Knight )
I think the fundamental choice you would need to make in your system is whether you prefer a more diffuse bipolar surround field (popular for movies). If this is the case then I would suggest a pair of Gem XL's (or the UIW RSS series)--they have the same woofers/tweeters as the Ones. If you prefer a more directional approach, a pair of 8's would be superb, followed by a pair of 2's.
On the other hand, if you really emphasize surround music in your system, then directional speakers are probably the way to go, so a pair of 8's would fit well (or a pair of 2's).
Just my two cents...Good luck!
Addendum: I should have read the title of your post before I replied--prefer Gem XL's over the Gem's for this application--worth the extra money in my opinion.