S.F.
Fl law states a child 3 and under must be in a car seat in any type of motor vehicle excluding public buses. But a child 4-5 may use a car seat or just the seat belt. Most taxi cabs in FL carry car seats in their trunks. The law requires you use one but I have heard some taxi drivers have been known to "look the other way". Not that I would recommend this. If you feel comfortable using a car seat provided by the taxi company then I would go w/o it. It is less to carry.
I have never used a taxi with my daughter but my husband's family lives in Fl as well as many of his friends (and he lived there til he was 25) so we are familiar with how things work in central and south Florida. Hope this helps. Have fun in Miami (I just got home from there 2 days ago!)