Really, he's warning you that corals do far better in a more established tank. Most people estimate about 3 to 6 months before adding corals, allowing everything to get settled in and macrofauna to grow. But, mushrooms are fairly hardy, so, along as you keep up with your maintenance, you shouldn't have any bad issues.
There's one thing I'm truly concerned with. There is a small thing with long polyps on the right side of the tank, about midway up. I can't see if that is aptasia or not, so you're just going to have to ensure that it's not. If it is aptasia, go pick yourself up a bottle of Joe's Juice from your LFS, take it home, and nuke that sucker before it spreads! However, if it's a coral, enjoy it!