Private guided tours
 

7 friends in Rome together. Flying into Milan then by train to Venice, Florence, then Rome. What would be best way to get a private walking guide? Book one from US or wait until arriving in Italy? Any recommendations for specific agencies?

I like Context Travel. You can book it before leaving on your trip.
www.contexttravel.com

We have enjoyed two tours with Katie Parla in Rome. I would recommend. My husband likes beer and is enjoyed getting to know craft beers through Katie. http://katieparla.com

We used Daniella Hunt....she is wonderful!
http://www.rome-tours.com

I second Context Travel, available in many cities with knowledgeable local experts.

Highly recommend Walks Inside Venice http://www.walksinsidevenice.com/who-we-are.php.
Contact Roberta Curiel. She is a life-long Venetian. She and her team do a wonderful job of showing you Venice in all its glory.For a group of the size of yours she will probably customize a tour around your specific interests. She can also probably arrange tours in Rome through their affiliates.