Since Bethesda has made the Imperials a race separate from the Nords then I would say it applies to Tamriel as well. If the High King of Skyrim is below the Emperor in an org chart then the same rules of Empire apply to Tamriel. If they are both equal then, technically, it's a confederacy.
The imperials as a race are a much different idea than the Empire per se. If this was only a matter of another race subjugating Nords, then totally, the choice to join the Stormcloaks is obvious.
This is unfortunate, that they're named alike. Imperials are merely a race of men from southern Cyrodil. They're not the empire. And the empire are not "outsiders" even in the slightest sense of the word. They are not. I can't emphasize that enough. The empire has existed for hundreds, nay thousands, of years in various incarnations. All of which included Skyrim. The empire is an idea started with all humans and elves in mind. I'm not sure what race Alessia was, but this game world wouldn't even have the concept of it's gods without her. She blended both elven and human cultures and dieties, didn't quite play favorites to anyone, and made concessions for both Nords and elves, and she created the foundations of the empire by organizing this network. Down the line, Talos took the empire concept further, and he was said to be either from Atmora or High Rock (either Nord or Breton probably). What both Stormcloaks and the Aldmeri Dominion represent are two factions that want to dismantle the work of TES's greatest "saint" (Alessia) and greatest hero (Talos). They're both isolationists and can only define themselves by their differences and waging constant war. This was what the empire has always tried to silence.
As for the typical idea of a "king" - it hasn't existed in Skyrim for milennia. They've always been under the authority of the empire for as long as I've pointed out above. The idea of a self-governing high king ruling Skyrim is a concept that no one has any reference of or experienced personally. It's not like they've just been invaded and a bunch of old men are saying, "Well, in the good old days, when Skyrim ruled itself, and we had a king.. blah bah blah.." is silly. Those good old days never existed for long in the first place. It is so far back in their history that it'd be like if I got nostalgic about the "good old days", when tribal chieftans ruled the American landmass, circa 1800 BC.
edit:
Sorry, that was a mouthful for such a small quote
