TG4,
The rule of thumb I use is 100 pounds of predator fish per acre. The question I don't know how to answer is how CC's or even crappie, count if at all in that rule of thumb. My experience is with LMB and HSB.
One other thought...if the CC's are primarily for eating, if removed and replaced by HSB you could get much better predatory action on the crappie, simply excellent sport fishing (far superior to CC), great eating fish, and no worries about overpopulating. I am just leary about stocking them with the 400 CC's you already have. If you are set on having CC's in your pond, then I think I would not add HSB...but having said that, I would much prefer HSB to CC's in all respects for your pond.