I don't think the redguards are quite as bad ass as some claim them to be. They were able to fight the invasion force in Hammerfell sure, but let's not forget the main AD forces marched on Cyrodiil and were focused there and the Empire pushed them out as well.
...with help from Nord and redguard legions.
An alliance with Hammerfell has to happen, but we're talking about Hammerfell also because they provide an example of how the empire now sees its provinces- as barter coin. They're also an example that if you stand and fight the Thalmor and don't give in to their manipulations, you
can beat them. The Thalmor are better at the political game than on the battlefield. The empire is trying to play their political game and I don't see that going well.
The human lands are going to have to work together, for sure, but the legion is no longer functioning. They're crippled and flailing. Ulfric didn't set out to be a revolutionary- he refutes that several times in his dialogue- rather he sees it as something that he has to do or Skyrim goes down with the ship, most likely first to keep Cyrodiil afloat. The question is not whether Skyrim fights the Dominion alone or with allies, it's whether Cyrodiil still has the right to be calling the shots for other nations any longer.