Funny, because last night I was staring at my tank and I also noticed tiny microscopic white things that were moving too fast to be baby snails. It's the first time I've seen them. They are much easier to spot on really dark soil (I had reddish/brown gravel before). A magnifying glass didn't help very much either. I wonder if that's whats in there. Interesting. I recall wondering how many tiny little microscopic critters there are in there.
I'm also about 3.5 weeks into my fishless cycle. All I've added so far is plants that I acquired through SNS and a a couple from my LFS. Pond snails showed up after that, so then I added 2 assassin snails. That's it. I just assumed they hitchhiked in on something.
Also, both of your links go to the same video.
I am guessing copepods.