We know (from DS9 and Voyager) that in the 24th century, gravity can be localized, so everybody should be pretty much fine.
Hey, gravity was localized in ENT already, as per the "sweet spot" and Archer's shower incident.
And in TOS, for that matter. If a shuttle can have its own gravity, why couldn't Kirk's cabin?
That said, I'd be surprised if the Romulans didn't look for a planet that had similar gravity to Vulcan's. I mean, given the option--and in Trek there are clearly tons of options for uninhabited but decent planets, and probably more back then--would you settle for a planet with .7G? Sure, it'd be cool for a while, but you'd wind up all atrophied.
Probably they didn't have that much choice. And who knows, their lush and watery world might actually be a hellhole for Vulcanoids - the equivalent of a putrid swamp to a human? Refugees or fugitives might end up in all sorts of sewers, especially if their transportation didn't have much speed or range.
On the general issue of strength, I doubt that fights are a good way to establish muscular capability. The strongest man on Earth might fall for one well-placed left hook from a weaker than average challenger, because muscular strength doesn't exactly protect one's jaw. If two humanoids wrestled... Now that might tell us something. And wrestling is exactly what was established to be hopeless for a human in "Take Me to the Holosuite".
Was T'Pol ever wrestled down? By somebody who couldn't wrestle down Archer (as opposed to a villain who got bested by Archer because our heroic captain punched him first)? I'm not all that familiar with ENT yet, having only seen a fraction of the episodes.
Timo Saloniemi