I think it depends on what "nearly fully stocked" means. Do you have a list of livestock?
You'd be adding about 30-35cm of fish, so space is one issue. However in a 400gal you probably still have room. The bigger issue for me would be demeanor of yellow-tail damsels. I've had one and they can be quite territorial (probably less of an issue since they will be added last). Nevertheless, you don't want them to bully other small docile fish.
