I think you should be fine. Remember though, that just because you have the Fantasmic dinner package doesn't guarentee you seats- If those reserved seats run out, they run out. I've read trip reports about how people were turned away from the reserved seats and had to stand in the 'standing room only' area. As soon as you get out of dinner, I would head over to the theatre.
With that said, I love Mama Melrose and we have the package when we go to WDW in September!
Have a great trip!!!