Just keep an eye on them. They are generally safe to keep next to each other. I recently had 2 torch colonies, 4 frogspawn colonies, and three 4 hammer colonies all touching each other without any problems. However I had to relocate a small torch colony because of his sweeper tentacles, he was trying to attack the other euphyllia's. I didn't see any damage to the other colonies, nor did I see any of his sweeper tentacles stick to the others. I relocated him to be safe.