Hamel should have defeated Chestra with Aerith's theme
I'm leaning more towards the Giant Chocobo Summon. Sephiroth only heeds the One Winged Angel summon song, you know.
And, since Raiel's playing along with him, maybe it'll fuse with the Firebird and become Giant Fried Chocobo Summon, which Hamel and Zack would promptly devour, of course...