Welcome to the Shack.
1) LCR speakers should be mounted such that the tweeter is level with the ear at sitting height. Don't worry if it has to be a little higher or lower due to screen or stand or whatever. Just make it close.
2) For 7.1 two speakers should be to the sides of the listening position and two in the back. If you have 2 rows of seats slightly behind the first row (on the sides) is probably fine. I'd need a picture to recommend anything more specific.
3) If you are wiring in wall, make sure it is CL2 or CL3 rated. Local codes vary, so if you aren't sure, check. Mostly CL2/3 is rated for almost anything you can do in a house, so it's a safe bet. As for brand, Parts Express has their house brand, Carol, and some others. Ram Electronics has some bulk wire as well. Some people like the Belden alarm wire for speaker use. The most important things are strand count and overall gauge. For longer runs, use 14 gauge or 12 gauge. Personally, I like 4 conductor 14 gauge and use two together to make ~11 gauge (I think I have that math right). If the runs are really long, consider 10 gauge, but I wouldn't go nuts unless you had to.
Avoid snake oil companies like Audioquest, Monster, and the like. Google or search here for more information. You're just spending more money on marketing, not performance.
4) I've had customers rave about HTDirect.com and AV123.com speakers. They both have several lines to choose from and the fit and finish is phenomenal. They sound great too. You're looking at $1500 to $2500 for seven speakers, though. There are lots others, but those two are a good start.
Good luck.
