Kin'emon would just easily get destroyed if he fought Pica and that should be obvious. It seems as though Pica is Mingo's strongest of the three. Kin'emon most likely isn't going to become a Straw Hat so him fighting is out of the question. Zoro will never lose to another swordsmen. Fuji already said he was going to deal with the citizens, he's most likely gone already, even if he is not I highly doubt he will get in the way. Kin'emon can follow Luffy and take care of Law or just watch that sexy fight. Luffy heading in there alone is just sweet if you ask me. He can take care of himself, nothing bad will happen to him. I wonder if Usopp will get to fight that other sniper, Gladius. Robin should be the one to deal with someone such as, Trebol. That would be fantastic.