Theres quite a large number of specie that will release terpenes into the water column, that regardless if there on the other side of a 300 gallon tank they WILL be subjected to it. The effects will vary, from out right recession, to discolorment, to lack of any significant growth, and or, all of the above. People that do mixed reefs more often than not, find these effects can be subdued to a degree on the most part with carbon.
Bottom line it comes down to your own personal definition of what success is. Is it just the ability to keep them alive amongst each other? Or is it for maximum growth? Or coloration?
I define success on the latter two. The best way to achieve this is to emulate the environment from which these animals come from, whether that be critters or corals. With that in mind, softies arent found in the same environments as SPS. For example you wont find thickets of acro colonies living in harmony amongst corallimorphans, and leathers, and vice versa. The terpenes or "toxins" released are part of the reason for this. It's a form of protection, and territorial aggression, certain macro algae release this as well for the same reasons. Hope this explains the why's a lil better.
-Justin