I think the relationship between Shank's and Mihawk needs to be unwrapped a bit more. Seems that Mihawk went to the meetings with the other Schichibukai (sp?) in order to gather information for Shank's. Out of all the warlords we've seen, only Kuma and Mihawk were able to talk formally with the others and from what we may conclude, one of the warlord's, Kuma, has had ties with an external organization (Revolutionaries). Mihawk may have had been in Shank's crew you never know. Or possibly, they built a freindly relationship from their sparring matches over the years. I wonder why Mihawk stopped sparring him, even though it's stated that Shank lost no power even though he lost his good arm.
Mihawk may join Shank's crew when they possibly battle the BB pirates I have a hunch that they will both meet their demises then and there and their defeat will echo the seas causing a rukus. Shank's may have military strength, we haven't seen anything in regards to Shank's crew except Rockstar, Ben Beckman, Lucky Roux and Yasopp. We haven't even seen their utmost power nor do we know if they have any particular abilities. In addiiton, Shank's seems as if he does not want anything to do with One Piece, I think he's just there to usher in the new era and try his best to dispose of any interuptions that will affect it. He seems like the man who will only act when provoked and does not act on a whim. Everything Shank's has done thus far, has been for a reason. In regards to the WB Pirates, I'm more than certain they have all alligned under Shank's name along with the other allied pirates.
As for Kaido, we know didly squat about him as well so we will have to wait and see. From what I'm guessing he has quite the military strength since he has been around for a while. As for the Shank's encounter, I think words were just exchanged, if it was an all out crew fight I doubt Shank's and his crew could be at Marineford in a day. Kaido is an obvious opprotunist no doubt and he will expose any opening he can and use it to his advantage.