If this is your first tank, then I would follow that rule. you want your tanks perimeters almost perfect for the nem.
Don't get me wrong, you can add coral, but you will also want your nem to be happy and go where it wants to live. Just be ready to aquascape incase it wants to reside on your zoas or frags.
Here is a shot of my tank I wool today.. I have a few nems in there and not mush corals yet.. I am waiting on everything to start being happy in their place first.
Sorry about the lights and shadows, I just got a new light and starting it off low to acclimate everything so I do not burn the tank.
The one bigger nem rubs against the zinnias and they do not seem to mind one bit.
I have 3 BTA nems, a condy, and a few rock flowers in this tank.