close isnt very descriptive is it?
if you moniter the flow of your tank you'll know wich way is "down stream" Dont put one leather downstream from another as they fight via chemical warfare. 4" is about the closest I would put any leather to another and thats pushing it. Honestly I dont have a comprehensive list of what can and cant go near them, basicly any sinularia fights the same way and shouldnt be close to another one. it wont bother mushrooms or zoas, or palys to my knowledge, I'm pretty sure most LPS are safe as well as long as they cant reach your leather they will sting it, if they do sting it it wil release more toxins as a combative response to the stinging.
(as a side note I have a sinulria dura growing 2" under the cap of my huge toadstool leather) not reccomended but its working for now. its also upstream persay.