I will never again accept a touring partner who tries to join me in the middle part of a trip. The rush I had to do to meet Steve in the middle of the trip did not do much for my sanity. Furthermore, someone coming in the middle of your trip has a very different mentality than you do. You might be worn out from riding many miles already. Or it might be that you've settled into a rhythm that's very different from that of someone who just stepped off a plane.
Furthermore, on a European trip, there are very good reasons not to take someone who's going to spend less than 2 weeks there. A "short-timer" is under more pressure to see more and get more done. That is incompatible with a relaxing bike tour. Furthermore, a week of bad weather wouldn't ruin the experience for someone with 2 weeks of vacation, but someone with just a week may, due to bad luck, simply get a poor experience.
Next, I would have to camp out with or spend many weekends with someone whom I'm bringing on the trip. It's just too important to understand each other's touring habits to not do that. Problems that may not surface in riding on day-rides will surface when you camp out with a person, and it's better to find that out before you commit to a multi-day ride with someone.
My experiences on this trip point to the ideal touring group size being 2: having an extra person to go get help if one of you get in trouble makes good sense, and with 2 people can squeeze into lodging that 3 or 4 cannot, which simplifies touring logistics.
One last observation: this is the first time I've done a tour solely with men. On previous tours, either I was solo or there was at least one woman in the party. The big difference between men and women when touring (other than the difference in physical abilities) is that if there's no conversation the woman's convinced that the situation is tense and that the man's mad at her. With two men, entire days can go by without much conversation, and there'd be no pressure to talk, other than the "time for lunch?" type conversations. It really is quite different (and for me this time anyway) and refreshing.