stang, as you mentioned, many of your smallest life will enter your ecosystem naturally, especially the macrophytes and planktons. Your "carrying capacity" will be whatever the pond can support based on what can consume nutrient present, and in turn be consumed by higher life forms. If 2 species compete for one food, the total weight supported will get split between the two...say 2 shrimp species eat the same food and that food supports 10 pounds of shrimp....you will always have ~10 pounds, regardless of how many species or the individual sizes of them.

For any given fish to thrive and grow, it needs quality foods, AND, in the proper sizes. For example, a LMB needs roughly 10 pounds of food, in the optimum size to gain one pound of flesh. If that LMB can hunt/eat 10 pounds of food, 10, one pound meals will burn far less calories than if it has to hunt/eat 160 one ounce meals....The smaller meals will result in considerably less weight gain due to it expending far more energy capturing it's meal.