One thing I'll point out is that the scale you're using will be misleading to people who are familiar with it. A "D" is generally defined as about 9-11 mph average. What you're calling a "D" is generally considered a "B" (11-13 mph average, with 15-18 or thereabouts on the flats) and if you're doing any kind of distance, will take a pretty strong rider. Generally, it's tough for groups of different abilities to ride together unless you have a leader per ability level. If you can't get that, you can make it a "no-drop" ride and wait at stop signs, traffic signals, etc. for stragglers.

But different clubs interpret the letters differently and as long as people know what to expect, you'll be OK. Another good alternative is to pick routes that can serve as "out and backs," meaning that there are minimal turns and even the slowest can know that they'll be met by others who've finished the route and are on the way back.

The biggest thing to make sure you do when leading a group ride is make it clear whether those who can't keep up will be dropped and left on their own to get back or whether it's no-drop. Setting expectations will help avoid confusion and troubles.