I find that it's pretty easy for riders of different fitness levels to stay together when it's flat, but it gets much harder to stay together when there are a lot of hills. It's difficult for a strong climber to match the pace of a slow climber going uphill and even if the faster rider just coasts on the next downhill there's no chance for the slower rider to ever catch up. In order to stay together, the faster rider has to actually stop and wait at the top of the hill for the slower rider to catch up. It's ok if there are just a couple of big hills, but if the terrain is a lot of up-and-down type hills the faster rider has to stop and wait every few minutes. I have been both the slower and the faster rider in those situations, and it's hard for both, which is why on normal brevets people generally just agree to split up for a while. I have never ridden a Fleche, but if I do I would like to do it on a team with pretty close climbing ability if the course were hilly. I wouldn't care as much about matched fitness on a flat course as long as everyone had a good attitude.