My advice to your last question is, if you have different interests, then plan on doing some things together, and some separate. You need not be joined at the hip for every minute of your trip, but make sure that everyone understands that. I have travelled with a group of friends where sometimes we are together and sometimes not. This way they can shop, or go running, or whatever, and I can indulge my passion for obscure historical sights.
Some compromising will be necessary, but everyone should get something out of the trip.
What interests you in each city?