I have been on 5 supported tours and on none of them were we expected to all be at the same speed. Sure, there was a leader and a sweep, but there was a very far distance between the 2 (or 3). Most tours have several routes for the day, within a given level, and you can always choose to go in the van. It's not a group ride. Backroads has several trips at level 2 or 3 where this would be a good option.
The only tour I went on where we were more of a tight riding group (with some days different than others) was when I went to Spain and there was a group of 10 (smaller than most US tour companies) and we just happened to be of similar riding speeds. They passed me on the downhills and I passed them on the climbs .