I don't know what the extent of Shikaku's plan is. It might have been just to separate Madara and Obito from the Juubi, or it may also be to actually defeat the Juubi. Either way his plan will ultimately fail, so it will become necessary for the Hokage to join the alliance. I'm assuming that the plan will fail because the Juubi's next transformation will make it more powerful than he predicted, and they will need the Hokage and Sasuke (+ Taka?) to win.