Amphipods, or scuds, as they are often called, can number in the hundreds and still be barely visible. Look right at the substrate level, they tend to 'hop' from spot to spot, it's a survival technique they use. They also have some assorted colourations, which makes them harder to see. But if you put 200 in there, they are still there.
I had them get in with plants, just a half dozen or so and despite a half dozen cories and two dozen kuhli loaches hunting them daily, they flourished in that tank ! Copepods, I'm not so sure would thrive, but scuds, for sure, they are tough little guys. They are also excellent algae eaters. They'll eat live plants if they are hungry enough, but so long as they find enough detritus or algae, they leave live plants alone. Mine never bothered any of my plants.