Most fisheries biologists and pond management folks will tell you the pond should be a minimum of 2 acres in size, and preferably 5 acres, before considering stocking crappies in it.

The urban public ponds I fish frequently seem to be in the 2-acre size range. Just my guesstimate, of course. The DNR initially stocks them for the City, with bluegills and catfish the first year, then they add largemouth bass the second year. It appears the City has stocked a few grass carp in most of their ponds for weed control. I don't believe the DNR stocks crappies in these ponds, and yet almost all of them have some sort of crappie population...anything from just a few stray fish, to effectively overpopulated with crappies. MOST of the time, if there are good numbers of crappies, they don't get real big. And the bluegills don't get very big either. These public ponds are further hindered by folks harvesting any bass that reaches decent size, as well as the catfish. This eliminates most of the natural "crappie population controllers" in the ponds.
I can't control what other people harvest from these ponds (and they DO get a lot of fishing pressure), so I release everything I catch. Whatever Rick is doing on the ponds he fishes...seems to be working very well!