Nitrites are NOT more toxic than ammonia, but they are still dangerous. You want your bacterial population built up to where it's turning ammonia and nitrite into nitrate as fast as it is produced (meaning even though the livestock creates these chemicals, in a cycled tank you will never see ammonia or nitrite in your tests.) It has been recommended to add hardy inverts such as snails first, and feed them for a while until the system catches up with their bioload, before adding fish. Not a necessary step, but an advisable one. Add only one fish at a time unless they are either tiny or require schools, and watch for your parameters to settle again before adding another.
In a reef, always quarantine anything that's going to go in, because once you introduce a pest/parasite/disease it can be nearly impossible to eradicate it.
