Be aware that the first-gen XMC-1 had a HDMI board non DHCP and HDR compatible. Emotiva now sell a new version with an upgraded board or you can have older unit upgraded by Emotiva. Cannot do that on your own.

I have never owned or heard a system with one, but reviews and feedback by owners is overwhelmingly good.

Emotiva had many choices for 7-ch amps, just have a look at their website, Other good value alternatives are Outlaw, Monoprice and ATI.

Many people use a combination of separate amps for LR mains and the remaining channels, so, yes, nothing wrong with mix and match. A prevalent line of thought is that you want to have the L, R and C with the same type of amp and power and matched LCR speakers, to have a seamless audio "front". Not so critical if you care more about 2-ch music and HT is secondary.