What type of Key West tree is this? It has beautiful flowers!!?

Yes this is most likely a Mimosa tree flower though it looks rather large. Could be the photo, not much for scale. If you remember the tree having a fernlike folliage then yes it is a Mimosa. The bad news is that there are some strictly tropical trees that have this same type of flower and will not grow outside of zone 9 or 10 and this may be one of them. The good news is that the Mimosa tree is hardy all the way to the Ohio river valley (zone 5 to 6) and has almost the same flower though maybe not as large. A word to the wise, DO NOT plant near a concrete walk or patio or near where you park your car. When the flowers drop they stick to almost any hard surface and are amazingly hard to remove. Plant in a grassy area and avoid this problem. The leaves are also very nice and like a fern and sort of a chartreuse green. You can even get a new variety with a purple leaf, very pretty, but the pinkish red flowers don’t show up as much against this variety. I love this tree but some people hate it because it can make a mess if planted in the wrong place and can also spread by seed. I have never mided just pulling the few new trees by hand, but I am in zone 6 and I hear in warmer zones it can become a full time time job. I say give it a try, it spreds out with curvy branches much like a live oak (of course much smaller) and evokes the feeling of a japanese garden.