I have a Syno Eupterus in my community tank, hes larger than all the other fish i keep in there but hes harmless and likes to hide in a cave all day, he sometimes squabbles with my upside down catfish (whose a lot smaller in comparison) over resting spots in the cave but nothing menacing or too aggressive, they are nocturnal so if you have hiding spots dont be surprised if you dont see him much during the day. I have Cardinals in my tank (quite small fish about between 1-2 inches) and my Syno (whose about 8 inches or so) would definitly be able to pop one in his mouth if he wanted to, but that hasnt happened and ive not seen any aggressive chasing from him either.
The Syno will grow quite large, and they can live for about 30yrs or so. Its likely i'll have to rehome mine in another tank with larger fish at some point, however if you dont have that option then dont buy him unless you are keeping him in a community tank of large fish, my experience is they grow quite fast in relation to how long they live.
Hope that helps