that tank should be good for upper jaw and should be good for most Lower jaws if you keep a limited amount. Polypterus in general will eat anything that can fit in their mouths. So if you want to keep the synodontis you have a couple options with polyesters. 1) you could just not keep them (which I do not recommend). 2) raise some smaller polys with the synos and take your chances. Or 3) Keep a smaller species. If you want to keep polys and synos I would recommend are senegals and mokelembembe. If you raise them small and wouldn't mind taking some chances I would recommend all of the Upper Jaws except the teugelsi, ornates, and weeksii. I would especially stay away from Lower Jaws if you want to keep polys with synos because lower jaws usually get bigger and are more likely to eat anything that can fit in their mouth. With the upper jaws they are more of the type that if you keep them well fed they should be a problem.