Any rules with the use of the Imp in the pen, as in certain things cannot do?
Good question. I always felt the original incarnation was the most powerful being in existence but limited by the fact that his master, Johnny Thunder, was kind of a dim bulb. However, DC Fandom has this to say: He cannot be ordered to kill, he must interpret commands literally so you have to be careful how you speak, and as an electrical being he is vulnerable to water.
They also add that his summoning word, "Cei-U" pronounced "Say You" (or "So Cool" is close enough) is actually what his name Yz sounds like backwards. So can he be banished by saying his name forwards, sort an inverse Mxyzptlk?