Just for the record, the northern most key of the Fla Keys is Soldier Key but it is not considered a true Florida key since it is not made of ancient exposed coral. The northern most "True Fla Key" is Elliott Key.
True Florida keys, exposed ancient coral reefs
Old Rhodes Key
It's all very interesting. The Florida Keys are categorized based on their geology, thats where the upper, middle, and lower names come from, changes in geology, create the divide. Soldier thru Sands are considered transitional because they are sedimentary in nature, but not really, and just walking around one you'll see what lies beneath the sandy stuff, fosilized coral reef, just like Elliot. I'm trying to think if there's hardwood hammock north of Elliot, hammock seems to be a big definer of how significant a key is. One site I found also had Key Biscayne listed as a FL Key.
I got to do somthing better with these days off from rain...