Not that I ever come across of. When small, ecats may tolerate many or even most tank mates IME, but when bigger, I'd not house any other fish with them, albeit a few exceptions, like synodontis catfish have been reported.
Is this a general consideration or you can support this statement with your experience or that of others? AFAIK, size doesn't matter much, even a 3" gulper won't be eaten by a 6"-er or an 8"-er. It's been reported that gulpers may mouth other smaller gulpers at making acquaintance but spit them out soon.
Also, what makes gulpers safe in groups is not the similarity of size, because they are specialists of swallowing a prey that is bigger, sometimes amazingly bigger than they themselves, so size is of no assurance. What makes them safe is the pronounced lack of cannibalism.
You can find them in strange poses and positions at times when they are resting.
