High Rock would want the Reach, which was given to Skyrim by Talos, back and they may not be too entertained by Ulfric's buthering of Reachmen which are a Breton people.
Nords don't seem to like Redguards much for some reason (the "Warmaiden" says she would have been kicked out of Whiterun if she were not married to a Nord and youwoul expect them to treat the anti-Thalmoric Ali'k better as well) so doubt these would two ally either.
And lets not start about how in history Skyrim was constantly violent towards these two races, ALWAYS attempting to spread its borders over their lands (and in other times helepd Cyrodiil conquer them). High Rock and Hammerfell have little love for Skyrim.
Redguards and Bretons may actually form their own alliance should the need arise (their relations have been getting better throughout history) and the mix of Warrior/Mage nations would make them a formidable defensive against the Thalmor. And since they don't need Skyrim in alliance and both have lands Skyrim took from them... it is quite likely that Skyrim would get invaded by these two... especially after Civil War weakened it.
The Reachmen are Breton descendants. What do you mean by Ulfric's butchering of the Reach people? Those who resisted? I don't think that the Empire put him in charge of executing and torturing the natives. His job was to reclaim the Reach, the Empire did the rest. Most of the butchering is now dealt to the Forsworn, who are extremists. And let's not forget that the natives of the Reach are Daedra worshippers, I wouldn't be surprised if the Bretons treated the natives too kindly, either.
Um, what exactly are you getting at? Adrianne Avennici isn't a Redguard, she's an Imperial. Where ever did you get that diea? Here's a quote from Amren, an actual Redguard "...our kind get along fine." This is said to the Dragonborn if the player is a Nord. In addition, the Alik'r were sent to Skyrim in secret. The King of Hammerfell (or whoever) did not write a letter to the Jarl of Whiterun or anyone what the Alik'r were doing. All that they will even tell you is that they're looking for a woman. And they were allowed into the city, they were kicked out after an 'incident.'
Redguards and Bretons won't make their own alliance. Hammerfell is renounced by the Empire. Unless High Rock secedes, Hammerfell will not forge an alliance with them, to do so is to become allied with the Empire and the Thalmor. This is almost as absurd as the notion that the two would invade Skyrim. Is that a rational idea when there is a much more pertaining threat?
These nations aren't hostile to each other, do you think they'd hold a grudge over century-old territorial disputes. Do Germans hate French because France currently controls Alsace-Lorraine?
Hammerfell and Stormcloak-Skyrim have too much in common not to forge an alliance.
-Both have been forsaken by the Empire.
-Both are Warrior-like nations.
-Both have a distrust of Magic.
-Both despise the Dominion.
-Both are Independant.