First I am not an expert, but I did research YP diets first. Here are my ideas.
Plankton are not YP primary diet, larval YP eat zooplankton but soon add more insects, snails and fish to thier diet. Most of thier diet is made up of insects (like midge larvae), amphipods, fish and snails.
My guess is that the 2"-3" YP are part of a boom year class. Every 5 to 6 years YP pull off a huge spawn. That big year class suppresses the numbers of the next 4 year classes, because there is so much diet overlap between year classes. The initial growth of the boom year class is also slower because of increased competition.
Adding invertebrates like zooplankton is going to be hard with an established fish population. Zooplanktons defense mechanism is pure numbers, the only reason they survive is that they can replace 25% of thier population each day. If you add 100 daphnia into a pond it isn't likely that they will survive to produce many young.
Before I spent money on "pure culture" zooplankton I would find out how many and what kind you already have. A healthy population of zooplankton can actualy clear pond water by eating all the phytoplankton. And then I would find a lake that has good fishing (for walleye and pike, not for BG) and no exotic species, and harvest some zooplankton from thier.
