do you have a free shuttle service from port of miami (arriving from Carnaval cruise) to MIA to pick up rental car

There is, but the frequency is not good. There is usually a long wait when the cruise ship arrives. Its better to take an Uber / lyft that will cost you around $10

Beware, they only start after 8am and then we had to wait till 8:20am having stood waiting from twenty to. All the other car providers had multiple vans picking up while we stood there. I found them slow, the driver whistled at me any stuck his hand out for a tip having chucked my bags around. Wasn't the best.

Yes you do.

Yes, they do...tips accepted for driver (especially, big older guy named Juan -- he's fast, cordial, professional, knows his stuff n if u have to re-gas ur rental, he knows where cheapest station is!!)

Yes