Is this true if we are politely speaking to a man?
I don't think I'd use the feminine alta speaking about a man.
So, in speaking politely to a man, you would say "Lei è molto più alto di me." You are taller than I am.
Right, but according to my grammar book there is an exception with the past participle: "Dottor Biagi, l'ho vista ieri in TV" (Dr. Biaggi, I saw you on TV yesterday), where the feminine "vista" is used. I haven't seen that one on Duolingo.