Synodontis Euruptus are EXTREMELY peaceful fish. I had one in my 29gallon community tank, and the only reason he got moved to his own 10gallon tank is because he could get to the sinking pellets way before my 9 corydoras, so I had to seriously overfeed him just so that everybody else got some. He now shares the 10gallon with a molly and a killifish. He hangs out upside down in his cave for most of the day, and in the evening he comes out and swims around. Sure he would like a larger tank, but wouldn;t all of our fish like larger tanks, and larger tanks, and maybe even to be free in the wild with virtually unlimited swimming room... I would NOT put another synodontis in with him though.
Mine only took a year to grow from the size of a small corydoras catfish to the size he is now, about 4 inches long and big enough to make a quarter pounder out of, if you like fried catfish, that is.