The crew is level 21 now. Once they got to level 20 I left SFK and WC behind and zoned into Razorfen Kraul, a large and relatively linear instance with small groups of mobs and a handful of bosses. My shaman can still tear through the zone quickly and safely, and I expect this to be the case for a while. The downside to this is that my characters do not get to increase their weapon or defense skills, but this is mostly a concern for the paladin. Once we get to the point where I will have him tanking, I'll probably need to spend some time getting his skills raised to a point where they're useful. But that is still a ways off.
One nice thing with a group this diverse is the various means of debuff removal and crowd-control available. The shaman, paladin, and priest can remove poison and disease. The priest can remove magic effects. The mage can remove curses. And between the priest, mage, and warlock, I can crowd control beasts, humanoids, demons, elementals, and undead.
Point-blank AE abilities will probably be of limited use, and targeted AEs even less so, since I have to go to each window individually with the mouse. I'm still working on macros and am a long way from having a finalized and effective set of them. The hope is to be able to bind each classes' macros to a specific set of keys that allow some synergy. The ideal setup will allow the group to focus DPS on each target in turn, while the paladin keeps the mobs occupied and the healers provide enough healing to keep aggro manageable.
Another area where I will have to learn as I go is talent points. Talent specs that work for a solo character, or for a character in a group/raid, may not be as useful for a class that is being run as part of a multi-box setup. And since they're not really doing anything but standing still and soaking up experience, the expenditure of talent points right now isn't guided by anything aside from what I think may be useful in the future. My paladin has his points in the Protection tree, since I intend for him to serve as the group's tank in the future. Plus for now, the Blessing of Kings buff is very useful for the shaman. The others have points invested primarily in trees that promote DPS, with a focus on any talents that allow their spells to stun a target (to make up for the limited mobility of the group and make it easier for the tank to reacquire mobs that switch targets due to aggro changes). I figure that once they get to 50 or 60 I'll have a better idea of how to spec them for my particular needs.
Getting to level 25 in RFK looks as if it will require six or seven runs...