Even with incomplete team the battle still start. But to see Tsuna fighting his own dad in the very first battle is out of my expectation. Because Tsuna as the boss of the Reborn team can't afford to lose. But is the battle one-to-one? I hope so because it will be much more challenging.
As for Fon's team, I have a feeling that Hibari will be there. Such an awesome event (?) cannot without him, right? I think he will join in order to battle Mukuro.