I don't think Future will 'ruin' the metalizer finish, but it might change the appearance of it. Some guys are *really* picky about how their specific metal finish looks and putting future over it might make it too artificial or take away from the intended look they were going for. Likewise for a dull-coat. It will certainly change the way the metalizer looks, but definitely won't 'ruin' the paint.
If this appeareance or metalic sheen is an important issue for you, you should spray a test piece and see how it looks before going ahead on the complete model.
M.