You'll likely need more umph than one flash can provide as your mainlight (unless you shoot at a time with the ambient is low). You might want to consider something like the
Triple Threat. It's the best design for a multiple flash holder I've ever seen especially at a price I wouldn't mind paying myself. With it, you could put three flashes in the main umbrella and one on a stand with another umbrella for fill (then again, that would require you purchase another umbrella, I assume). As for a rim/separation light, hopefully you can put the sun to the subjects' backs. I use the sun for rim all the time.
Personally, I'm not so sure about using the 500L for family portraiture. My biggest issue with it is the subject-to-photographer distance hampers communication. I find fast, reliable communication to be key in nailing group shots.