I'm inclining toward a 6-shot load, Phil: more closely simulates centerfire realities. In any case, the rule will be that you can load only the number of shots permitted in that particular string: then you have to reload. No "high capacity guns", no "extra shots in case one malfunctions."
I also favor a short range (6 yards), since some members are VERRRRY restricted in the amount of space available: having to shoot indoors, or in a postage stamp backyard or alley.
As you say: go with absolute simplicity in terns of logistics. I want this shoot to be for the same sort of blokes who are already shooting in our BBS comps that PAD E. and POLLYTHEDOG are running: just average pistol plinker guys with a CO2 repeater and a few paces of space. I would rather leave the club shooters, with their long ranges and sophisticated electrical turning target systems, to run a separate competition. God love 'em, I wish that I had access to such a set-up, but I just don't, and I suspect most of our potential competitors don't either.
Jim