Running your 6 ohm rated speakers at 8 ohm shouldn't hurt them because you're actually pushing them less than they are rated for (higher ohms = less volume, which is less power and vice versa). You can ask your ohm-related question in the speaker forum where the experts hang out and you will probably get a more detailed and exact response.
Can't help you with the wall mounts cause I don't use them. Our speakers are on stands to keep them at ear level height which is best for small HTS.
I used 16-AWG, 2-wire ribbed for my speakers. The wire is actually heater cord wire which I purchased at our local hardware store for about $0.20 per foot. 16AWG is perfectly fine for runs up to about 30-40' before there is any loss of fidelity. Even at 35', I doubt if you could hear the difference.
Speaker placement can be critical for optimal listening. However, a lot of that is dependent upon room furnishings, etc (carpeted floors vs hardwood, furniture, wall coverings, windows, etc). Typically, but not always, the front sound stage (front R/L) speakers should be at least 6'-10' apart, slightly angled in towards the center listening position, and about ear level height when sitting in the listening position. The sub should be situated so that the sound doesn't seem to emanate from any particular direction. Mine, for example, is about 3' to the left of my L front speaker sort of in a corner. However, the sound, due to the furnishings, etc fills the room and when you close your eyes, you can't really tell where it comes from. Sub placement takes a little experimentation. The sides are equidistant on either side of the couch, at just above ear level and are about 2' further apart than the front R/L.
Speaker calibration can be a bit tricky as well. I had to manually calibrate mine because my receiver doesn't have the automatic Audyssey-like microphone-based calibration system. I'm not toyally convinced on how effective the Audyssey system is for small HTS but again, that would be a question to ask in the Speaker forum. Just keep in mind that no matter how you calibrate your system, once you move from dead-center listening, the acoustics are going to change. However, you should definitely calibrate your system as best as you can. Proper setting of the x-overs, depending on speaker size (drivers), can also dramatically affect your listening experience. Remember, listed speaker specs only go so far. You can't really compare speakers for audio quality based on specs alone because you don't know how the specs are determined (anechoic chambers, etc). If you can hear the speakers first, and mentally transfer that to your listening environment, that's the best. The bottom line is that no matter what anyone tells you, it's what sounds good to you that matters.