Its going to be hard to say. Each puffer really does have their own personality of what they will and wont put up with. My fahaka for instance will ignore my upside down catfish all day long. Just pretends hes not there, which is a good idea considering its double his size right now. However, if he gets bored, he likes to play a little game where he will sneak up on my cat and try to bit his tail. No damage has been done yet, but I am prepared for the fact that my cat will eventually have to come out. Your abei may let your synodontis live a peacful life, he may constantly nip the heck out of him. You wont know until they are both settled in. Just be prepared to move the cat or the puffer to its own place if need be.