Welcome 8worlds. You've got a very healthy looking featherfin there. Congrats. What you probably were observing was how Synodontis eupterus interacts when the lights are out and/or when they feel really comfortable in their environment. They really are a communal species given enough space, the right kind, and places to hide during the day. In the wild they have been observed in schools. It just hard for us as aquarist to replicate that environment in a small aquarium. So most of us are relegated to keeping 1 or 2 (if we're lucky) of them together. Not really being able to see their full social behavior like it is out in the wild.
I wonder if your fish is way to young to become pregnant, how long is the fish? You can sex your fish by looking very, very, very closely at the vent. If its pointed its a male; if its rounded like a soft mound its a female. Check out the description
I'm not aware that this fish has ever been bred in an aquarium. That said, nothing is impossible.
The big belly could also be a result of bloating. Fish get indigestion just like people. If thats the case you can try a number of things: 1) stop feeding for a day or 2 and see what happens. Don't worry they wont starve. Many larger fish can go days without feeding. 2) upgrade the quality of your food, and/or mix in some live foods like blackworms, earthworms, zucchini, spinach, sweetpotato. 3) check to see that food isn't just sitting or rotting in the tank and the fish are eating it later on. Good luck
