Well, let me see how I understand the group dynamic based on tasks we might face:
Infiltration is fairly well covered. Soot can sneak in, Dive Bomb can create a diversion, Crown Cork could probably grease his way in with his influence.
Dive Bomb covers medicine, and he and City Slicker can probably figure out navigation.
Grease Spot can probably figure her way around a trap, unlock doors, or figure out how to trap enemies.
The only real issue that I can see is direct combat. I'd probably be limited to chucking a stick of dynamite and then trying to snipe stuff with lightning bolts.
Yeah, and Grease would likely be limited to bucking or hitting things with a hammer. However, I think that given our backstories, none of us really should have experience with fighting (with the possible exception of Soot). It seems like the kind of thing that we would pick up with earned XP during the adventure.
How about social situations? I can imagine having to deal with unfamiliar and/or unfriendly people on the way. Crown Cork seems to be the most social, though since we'll probably be outside Equestria his influence wouldn't be much help.