Allow me to share personal experience here.
Yes, corals are toxic animals and that includes all corals in general be in softies or hard corals...and for a good reason - to protect themselves again external invasion by other species and or prevent being food by other animals.
In general , softies such as leather coral or zoanthids are view to be more 'toxic' as compare to other hard corals. There are reports that some hard corals don't do well when they are mixed with softies, this is not entirely true.
The reason why this happens is when the tank environment such as water parameters are off the norm causing stress to all tank corals, resulting in the release of toxic chemicals by a few could trigger a large scale of chemical warfare among the corals...hence affecting both the hard and soft corals.
In a properly maintained with good husbandry tank environment...it is absolutely possible to have both hard and soft corals hosting and thriving together. Carbon IS NOT NECESSARY.
There are tons of successful reef stories and pictures featuring both hard and soft corals living happily...key being a good husbandry.
- FANTASY CORALS