I've kept all kinds of Euphyllia together and they get along with each other - mix and match.
However, their sweepers don't even have to physically touch another coral to put a whammy on it. Anything that's downstream can suffer from chemical warfare. I had a nice Caulastrea once that was a good 6" past the longest sweeper's reach from a Hammer, and it started to wither on the side closer to the Hammer. Moved it upstream and the heads that weren't past the point of no return, bounced right back.
I love Euphyllia but one does have to plan placement rather carefully.
Jenn