do you absolutely HAVE to have speakers in the rear?
honnestly, coax speakers are crap, especially when compared to the Focals.
You should be fine running the focal seperates only on that amp, one channel per speaker. I haven't had rear speakers in my car in 4 years, and I will never go back to speakers in the rear. It's just not as pure as I like
Are you going for pure volume? or more for sound quality? it sounds to me like you are going for quality with what you have so far. True sound quality systems don't have rear speakers.
Now if you are looking for 5.1 sound then you will want rears AND a center channel.
There is a reason the HIGH ends of the HIGH end speaker companies don't include coax speakers....
but if you MUST you can look into the Polyglass (165CV) from Focal....they will take 70 watts rms
but, as well, just because your amp may put out 100watts rms per channel doesn't mean you need a speaker that can handle 100 watts, that's a common misconception
if you insist on using a rear speaker (if you are all they will be for is rear fill) I do recomend using the same brand as you will have the same voice matching ect....if at all possible use the same line in the same brand...
just my $.02 worth