I've been using Al IF it's in the spec, & occasionally for budget reasons. 750kcmil, down to 4/0. As to a ground conductor, Al is not a problem. Personally, I don't like to mix. I've seen #6 Al used for site lighting; had I issue where a 'nick' caused a leg failure in the pipe; not the fault of the conductor material though.
Bob & Don:
As to pentrox (etc) I've been using it for 30+ years! It's what my 'teachers' did, & old habits are hard to break.