Hi Eric. Welcome to HTF!
For HT use, soldering the speaker wires on the INSIDE of the wall plates is probably ok.
Some of the more audiophile wire sites say to never solder your speaker wires. There is no signal loss issue, but they claim it has an effect.
Since copper will oxidize, the solder will actually seal it off so you dont have to remove the plates every 2 years and trim/strip.
But consider this: pull enough speaker wire to create a un-broken run from receiver->speakers. Buy blank wall-plate covers and drill some holes to thread the wires through. This way you can always cut the wires off and install plates with binding posts later, but it gives you the benifit of a un-broken wire now.
You should run 12 ga for the longer runs. Most of us buy a spool of 12 ga and use it everywhere.
Be sure to run 3 sets of speaker wires to the rears so you are ready for a rear-center speaker that is becoming popular as part of a DolbyEx system.
Speaker Height:
You measure speaker-height from the position of the Tweeters (the small driver). You want the front three tweeters close to the same height, and you would ideally like this to be at ear-level while sitting.
I prefer the rears to be at the same height as the fronts. Dolby recommends putting the rears 3-4 feet above your head, but they still recommend that only ambient noises (wind, rain, low-car-rumbles) should go to the rears as in the ProLogic days. Many modern movies send specific sounds to one of the rears and it sounds funny to me if they shoot up over your head instead of swirling behind you.
So try both placements (ear level, 3-4 feet above) in your room and pick the one that sounds best to you with a favorite movie.
We have a
Beginners Primer & FAQ that will answer a lot of your next questions.
Good luck and enjoy your stay.