Our friend Lasse on the forum pointed out a neat idea once that stuck with me, the feeding we give to all the organisms is the hidden risk, not so much the animals. If we feed pellet food or protein meat chunks that aren't eaten but pile off to the side, they can overcome the system
It's ok to add animals but you are adding them slowly not due to lack of bacteria, or entry test kits that may or may not reveal accurate numbers, but so you can precisely feed and make sure not one pellet is wasted. If you add a few snails or crab, don't feed extra make them work off what you already feed the clowns. Occasionally offer the cuc a pellet but go easy on feeding for a while.
Research fish disease prevention options stickied up top in the fish forum, a tank of mixed lfs fish are 90% likely to die in six months.