whatever they are, I can say with ALMOST certainty they are harmless, and more than likely beneficial. Amphipods, copepods, daphnia, etc all eat decaying plant matter, detritus, un eaten food and algae. There are a few exceptions to that, and i think they all come in the form of fish parasites. But i think it highly unlikely that you would see them in such a large number if they were parasites.
That being said, I would think that its possible that having such a high number of them is an indicator of excess nutrients in the water...uneaten food or decaying plant matter. Or could just be that there are no fish in there eating them. But me personally....i think your tank looks great, and i am always a fan of higher biodiversity in an aquarium. The more things living in harmony together in an aquarium, usually the more stable it is.
Also, at this point, its not really ammonia you need to worry about. With all those plants, and only one fish, you probably wont ever see ammonia register on a test again. But without doing any water changes its nitrates that you may need to worry about. Nitrates can become harmful. I certainly am like you though, I do much less water changes than i should, and seem to get away with it just fine.