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.