Too many critters at once.... Means too many CUC members to be added to a new system at one time. The people who feed algae sheets have (at least I hope so) an established system, the sheets just supplement, it's not be their only source of food. Your plan is to completely feed all those algae eaters like they are fish that need to be fed, then there are the meat scavengers. Like Silverado said...competition for the food.
I have never had a fish hit by the spikes of a Diadema urchin (Black long spine)...but it's pretty weird to put your hand in the tank, and see them aim those spikes right at you. LOL...That red orb is not an eye, its an anus, so I don't know how it knows where to aim, since they can't see. I would move my hand one direction and then the other...the thing would bunch up it's spines and point them in whatever direction my hand was in.
A CUC is just that, their job is to clean up...but in a new tank, you have nothing to clean up. A CUC has to be built as the need for them arises. Urchins as part of the CUC, are what you add if you have problem algae, otherwise the snails do a fine job. You got in a hurry, and/or you wanted to "save money" by getting everything at once. This hobby is not cheap, and cutting corners like getting everything at one time, just to save on shipping charges, is how to crash your system, if you do that with fish you don't quarantine, your tank will be in real trouble.
I learn something new here all the time....I never knew that a serpent or brittle star would eat algae sheets, or any algae at all. Mine lived in the rock (never seen them) and feed on meat...like bits of food that falls in there that the fish can't reach or get to, when I fed the fish, I would see an arm reaching from the rock to grab a bit of floating food (they do come out at night, you can see them if you have moonlights)....I always got them because they clean up the wasted food in the rocks...and eat the dead, same with shrimp.