Velo, that's a great summary!

I think the ride distances will just depend on what time you start. If you do a 6pm start, a beginner/C group probably won't be able to do more than 15-25 miles before dusk. But if you do a morning ride, they could certainly go farther. I think for true beginner group(<12mph) 10-15 miles is good. Most of the beginner groups i've ridden with do allow any kind of bike (mtg, hybrid, road) to encourage more folks to come out, but that would be at your discretion of course.

Oh one more thing, define your requirements. Helmets, WATER! (some beginners don't bring anything to drink), flat kit, ID, emergency info....