I've gotten the Imp summon several times before. the first time I got it i was wandering around the the desert near the damcyan castle, there you can fight parties of Imps in groups of up to eight I believe. the last time I found it, I started a new game and was leveling up Cecil and Kain in the Mist Cave and lucked into it. I had not even gotten Rydia for the first time yet.

of the other summons, I know you can get them..but I havent ever gotten them.