Yes, it's true, especially in smaller tanks. The concern is that corals are aggressive animals, that defend their space. They sting competitors and will kill them. Some types are more aggressive, with longer 'sweeper' tentacles than others, and many also seep chemicals into the water for defense--this is easier to deal with, simply because the chemicals can be removed from the water with carbon.
The nudi is also a poor choice--for any size tank, as most of them are obligat feeders, meaning they eat only one specific thing, and will starve without it.