I love having a private guide in certain places. I love this because I
-- don't speak the local language well enough to do more than ask for a table or tickets
-- want to maximize my $$/time in a place where I'm language challenged
-- want to help contribute to the local economy where I'm traveling
And probably more, but that works for a start.
In Italy, guides must be licensed. One way I have found excellent guides around the world is to look at tours published by tour-oriented businesses (like Context, for example) which offers private tours in several European locations, including Florence. Sometimes I've used tour groups like this one to find a private guide in another location.
One thing that I keep in mind is that not everyone is as online/connected as I am. I try to allow for a reasonable period to allow the other party to respond. For me, if I haven't heard from someone within 5 business days, I assume they're not available or not as responsive as I'd prefer. That's just a me thing. That doesn't make them bad or me good. Again, that's just a me thing. But if I haven't heard from someone in that length of time, I assume they might not be actively pursuing business at that moment.