Well, why not? I mean Hammerfell broke off from the Empire and WON against the Elves. If the Stormcloaks ending is canon why wouldn't these 2 join together? Other than the truce in the main story being canon and ES6 picks up right after the main story ends.

Depends on if Ulfric is willing to accept help from another province and has the diplomatic skills necessary to broker an alliance. Even if that's the case, I'm not sure the Redguards would even want to help. They might still be recovering from fighting off the Aldmeri Dominion (not sure how many casualties Hammerfell suffered or how large their military is) and if so, probably wouldn't be in any shape to jump in and help Skyrim right off the bat. But if all that lines up, then hey, why not?

not to mention, historically the kingdoms of Hammerfell and Skyrim have rarely ever been "buddy buddy".. i wouldn't be surprised if such an alliance just created further unrest among the civilian population



if they did though, hypothetically speaking, I still don't think they would have the forces to take on the dominion on their own turf.. one of the main reasons the Redguards were able to repel the Dominion was because of the enviorment, the dominion just wasn't prepared for the environment of Hammerfell..

similar to the reasons Hitlers attempt at invading the Soviet Union fell apart, his people just weren't prepared to fight in such a climate (funny fact although somewhat off topic, at the time of WWII many of the allies military commanders believed the Canadian military as the only ones who could possibly successfully invade the Soviet Union, because of the similarities in climate)

I don't imagine that Skyrim would be in any position to support Hammerfell. Even assuming there is some sort of peace with the Empire and Skyrim forces could sail around High Rock, the just emerged from a Civil War in which many of fighting age were wounded or died. Also, even after Skyrim has time to recover, Hammerfell doesn't seem like the best terrain for a Nord warrior to fight due to the difference in home climate