to be honest i don't think it really matters the sort of fixer used ... i was always told to use speed fixer
mainly because, well, when i learned photo-stuff the teachers used sprint chemistry (speed fixer ), and not old skool hypo ...
but the more i read and think about it, it seems to matter less and less, and it seems a lot easier to use hypo than speed fixer ( low tech )
i think you are probably right about the cast, from what i remember whenever i got "it" i just washed the film a little
more and it went away, not necessarily over fix ... i was just being a parrot to be honest
" squawk use speed fixer to get rid of the magenta cast, squawk"