Its pretty tough to get anywhere with italian girls. Americans and foreigners are nothing exotic in Florence, in fact you're likely looked down upon more than anything. They are quite closed off in public because Italian men holler at every piece of ass they see, and they're especially creepy in clubs. Mostly when you're in clubs you're going to be dealing with international women who want to go clubbing and the hoards of creepy local men that they attract.
All the other foreigners are DTF though. The Lion's Fountain is the biggest Irish pub and it will be pretty much 100% english speakers (Americans, Irish, English people). Not the best place to go if you're looking for a hook up but it will certainly have plenty of people. Red Garter is pretty American/foreign, its kind of a hybrid bar with dancing, more likely to find a hook up there. Twice is a pure discoteca with a lot of American girls and a LOT of creepy dudes. You'll find a lot of DTF people but you also have to compete with those creepers and its effing packed all the time. All three of these spots are pretty close to one another on or right off of Via de Benci.
The one time I went to Central Park there was no line. Its kind off far out of the city center, but you'll find plenty of DTF people and less creeps (relatively speaking). Good mix of Italians and foreigners. Astor is right by the duomo, its American owned and kind of a combo bar/discoteca, lots of Americans.
Dunno about bar crawls, never went on one in Florence. Most of the places Italians frequent will have almost no one inside as they prefer to hang out on the street outside the bar.
