I've given it some more thought, and I think I can work it with just the Heresy minis. I just have to scale it down a bit, but I can then ramp it up by adding more cops later.
Comps are my only real motivator so if I'm ever going to get this done, I need the extra incentive.
p.s. props from other manufacturers are okay, right? The are necessary to set the scene - particularly the toy that just came in my McDonalds Happy Meal.