As for how many flunkies a dragon should have, I can see roughly three approaches to this:
1) Pick whatever you think would be cool. Just remember to include them in the encounter level to make sure the encounter is balanced for your party.
2) A Young adult red dragon would have (I believe) 7 feats to play with. Just pick Leadership as one of them! There's a little snag here, because Leadership score is calculated based on level. If you based it on ECL, you can easily wind up with a cohort that has a much higher CR than the dragon itself. If you base it on class levels (i.e. zero for a standard dragon), you would proibably wind up with a 1st or 2nd level cohort. Basing Leadership score on CR seems much more reasonable, since it preserves the assumption that cohort and followers are included in the leader's CR. For a young adult red dragon (CR 13, Cha mod +2) you'd get a 10th level cohort, one 2nd level and 20 1st level followers. Sounds about right...
3) For any creature, you can add flunkies totaling up to it's CR minus 2 without changing the Encounter Level. So the CR 13 young adult red dragon could have one CR 11 flunkie, or 2 CR 9 flunkies, or 4 CR 7 flunkies, etc... and the EL would still be only 13 (a bit over 13, actually, but less than 14). This seems to agree failry well with method 2).