I keep my Tire Track Eel (the largest of the spiney eels) with my CK with no problems. The CK is about 12", and the Eel is only like 7" or 8". They really dont pay any attention to each other, even in close proximity, and feeding. Make sure you have a good hood on your tank because these guys do try to get out!
I do recomend. Highly inteligent and interactive. Can be hand fed and even like to be stroked or pet. GREAT fish!