Quince (Membrillo) requires some dormancy hours in the winter. You should be OK north of I4, provided they get some PM shade and have the sandy soil amended.

The first set of seedlings mysteriously died - I may have been over-caring for them. Finally, another seedling come up, and I've not done anything for it so far, and it has about doubled the size of the seedlings that died already. Hopefully, I'll have better luck with it.

I've also ordered three quince trees: smyrna, orange, and an aromatnaya which should hopefully be here within the next few weeks.

One of the reasons quince is not prevalent in Florida is because (and this applies to many pear varieties as well) in hot humid climates the trees tend to be very vulnerable to fire blight, as well as other fungal diseases. Fruiting quince is native to warm dry climates—not necessarily desert dry, but it is not very tolerant of the intense Florida humidity.

Another issue, if a quince gets pollinated by a pear tree somewhere in the vicinity (like a flowering Bradford pear tree for instance), the resulting seedlings coming up from the seeds you planted will most likely not survive.

That being said, the amount of chill hours are not going to be a huge issue. While quince is less productive in anything higher than zone 9, it can still produce some fruit. There's even a variety known as "Karp's Sweet" that was discovered to be widely grown in home gardens in the Majes Valley of Peru (which is the equivalent of climate zone 10 or 11).