1)Wall thickness is not important as long as you do not go below the minimum. What is most important is the wall segments and hiding the texture on parts that are not visible.

2)1 quant= 1 inch... this was recommended for simple shooter games but was not a rule. Quants are more for texturing purposes then setting level size. Level size is determined by portal count and how many polygons. Using lod will allow larger levels. There is no rule to determine level size... it is your call.

3) 512 x 512 will certainly look better but can cause memory problems. How it looks when scaling is an important issue. 1024 x 1024 should be used sparingly. Texture size can change for each type of game. A quick paced shooter will need lower rez for speed issues but won't matter becouse the player won't be looking at the textures. An adventure/puzzler will need good textures but it may not be a problem since the player is not moving fast. So 512 X 512 may not be needed for a shooter. another issue with texture size is the download footprint... if you are making a game that will be downloaded.
The rule is use the lowest texture size possible that gives you acceptable results.
Only use a large texture size if it has a direct benefit on the game