I could not access your video. Please describe your tank start up, with special emphases on live sand & live rock additions. Also, I highly recommend you record your start up and use the thread as a tank journal, so that others can access information and spreak to specifics. With 53 years of Reefing, I could address your specific circumstances.
If you used real live rock: diver collected uncured live rock then you would have an explosion of micro fauna & fauna: yes to copopods & amphipods, isopods, etc, etc, etc
Maybe post the video on YouTube then just paste the link here. That way members can view the video from within this thread without having to download it.
I don't have an ID for the first two (I've seen the second before a couple of times, but it pops up so infrequently that I can never remember what its called or what thread I've seen it in), but the shrimp-like critter is a Tanaid (the pincer claws are readily visible in the first few seconds of the video).
Tanaids are generally scavengers eating detritus/algae, but a few eat worms and copepods.