This is a Splendid idea.
For something of this potential scale, though, I would say keep it as simple as possible.
Shooting for group size is easiest, as any target will suffice.
If shooting for target score, then stick to something like an NRA bullseye target at 25 yds for the handguns with separate rimfire and centerfire matches (having different categories for centerfire calibers could get cumbersome).
For rifles, you could designate a range for rimfire and centerfire and use an NRA target also, or for kicks, use a simulated sniper target. I found one that simulates a 'Carlos Hathcock' type shot at another sniper. The scoring zone simulates the other sniper's scope lens. You could go for tightest group here also.
Just my $.02 worth...