The young Smith in old mans clothes worked well with him, but I'm not sure that the old Capaldi in young mans clothes would work quite as well though. I don't want Capaldi to just be the complete opposite of Smiths doctor, and I don't want Capaldi to model himself on an existing doctor like Smith did.
Your concept could work, really I suppose I'd need to see a picture of Capaldi in the actual outfit to decide if it'd work or not.
But personally, the way I see Capaldi's outfit is just age neutral, where any age could pull it off.
Still, keep up the good work. :P