Welcome to the salty side! Take your time. I assume you are already familiar with the nitrogen cycle and have the proper teat kits for ammonia, nitrite, nitrate, calcium, alkalinity and specific gravity (salinity)?
I keep a slightly bigger tank (than your 20g aio) these days. A 40g breeder aio. I have lots of corals,lots of anemones, CUC and a few other inverts... and NO fish. At one point I had 4 tanks and a total of over 700g of saltwater in the house! But that was 10 years ago. I wanted an easy to take care of tank, thus the aio and no fish. Feeding the fish and the fish defecating in the tank makes the filtration system require more maintenance than I wanted to do. Other than testing and the once a quarter partial water change, I hardly have to touch the tank.