I had similar thoughts about a Romulan/Federation alliance after STXI.
Despite half-crazed military leaders, IMO the Romulans are a decent people, with a sense of honour (not in the Klingon "Smash! Smash!" way), and I think they'd be apalled at the idea that one of their own murdered the people and home of their ancestors.
The Romulans would be probably happy to conquer Vulcan and rule it, but I don't think they'd want them all dead and their world destroyed.
I can imagine them, once convinced that Nerada was future-Romulan, offering the surviving Vulcans asylum in the aftermath and palling up a little with the UFP. Or maybe I've just read too many sympathetic Rihannsu novels
Also, with the foreknowledge of the Hobus disaster provided by Old Spock and Nero to Kirk and Pike, I imagine the Romulans and Remans will have long evacuated come alternate 2387.
Also also: No Vulcan = no red matter = no way to stop "galaxy threatening" supernova in alternate future. Uh-oh.
The operative words here are: "galaxy-threatening supernova". . . so It doesn't matter if, or where, the Romulans move, they'll still eventually be toast. . .and don't be too sure that being forewarned of the Hobus disaster will be enough to move the Empire's "half-crazed military leaders" into action in time; realistically, it is extremely hard to overcome ingrained military inertia, to switch gears from conquest to building the infrastructure necessary to move billions off-planets. . . It is in the Romulans' best interests right now to ally with the Federation and help the Vulcans repopulate (so they are able to focus on more than rebuilding). . . but will they do it? Are the hostilities and distrust between the cultures too deep?