a standard 996 wheel is 5.5 inches and does fit a 180 section tyre well. 6 inch rim fits a 190 well but as for the 5. 75 rim, i don't think anyone here has mentioned the best fit for it. search for the wheel and tyre post by shazzam!
only through people who know what they are doing with Mag. its not as simple as other metals. it requires a special coating before being painted, otherwise it will all just flake off. best off contacting Dymag, although I found them to be a bit expensive