That's not true. Microscopic black holes only last a few millionths of a second before they decay. There's a little thing called entropy even black holes have to deal with. If they don't get "stuff" to make them bigger, they eventually go away. The size of the black hole is proportional to the gravitational pull and the distance it's gravity influences if proportional to that following the inverse square. So a small enough black hole would exert influence over an extremely small area, maybe not even far enough to influence a nearby particle only microns away. The CERN LHC particle accelerator should be able to create this type of short-lived, microscopic black hole when it comes on line this year.
We don't really know enough about Romulan technology to guess whether or not they'd be able to use their singularities as a weapon, but since they don't, I'd hazard a guess they they can't. The Romulans wouldn't be shy about wielding that kind of power.
