I would slow down a bit and let the tank mature and go through some 'ugly' phases. The urchins will not survive without sufficient algae (coraline, film, GHA). Leopard wrasses need a lot of microfauna to graze on. After my tank cycles, I usually put in a pair of clowns and try to add diverse microfauna and let the tank settle an mature for months before I add corals or other fish. I add snails and crabs as I go, but starfish and urchins need lots biodiversity of microfauna to live.
+1. A 21 gal tank with 4 fish, two of which should be in a tank of 50 gal or more, plus all of the other livestock is moving really fast. I would fix your white slime issue and get your skimmer settled in for a couple of months before adding anything else to the tank. Continuing to chase problems will only lead to more losses.;Nurse