I don't see why a mining vessel wouldn't have the gear we witness. Its main characteristic is that it's BIG, like a mining rig ought to be. And big means powerful. Nero could have endless stocks of 24th century firecrackers aboard, and quantity would make up for lack of quality - just as we witness. Other ST starships die from the impact of half a dozen torpedoes. The Kelvin is merely wounded by what looks like hundreds of small impacts, and then commits suicide.
Being BIG, the rig would also probably have vast interior spaces, most of which would have no role to play when the rig isn't mining anything. A torture chamber, two different kinds of roller coaster, and a sauna compartment with mood-enhancing firefalls wouldn't surprise me a bit, and might indeed be considered essential gear for a Romulan mining rig on a long duration mission.
The Borg thing is pure nonsense: if Romulans had access to Borg tech, they wouldn't waste it on a mining rig, and a mining rig couldn't plausibly go and steal Borg tech on its own. Anyway, the Borg are in the 24th century (or at least are locally keeping a low profile in the 23rd, and you can't find the Borg when they don't want to be found), and Nero left the 24th century immediately after Spock failed to save Romulus. No time to go hunting for Borg stuff between being a simple miner and becoming Vulcan's worst nightmare.
Timo Saloniemi