The Selan come in many shapes and indeed from many races. What unites them are their varied conditions and deformities and their sullen natures. They frequent dubious outposts, anonymous tradehubs and planets of lesser species. While rarely welcome for the upscale sellers, they find their place among the smugglers, trash dealers and other shifty folk. It is rare for them to be associated with an actual crime but few will care. To the law and everyone else they are firmly in the 'suspect' category for some reason. Some consider them bad luck. Many openly discriminate against them. A few know what they are and pack their bags when they see one. Those that find their way into the Alpha Quadrant find good custom with the Ferengi at least.
Preferring heavy drab clothing, the Selan like to conceal themselves. Those who know them prefer it as well.
Their spaceships are even worse: a hodgepodge of disparate pieces cobbled together, it's as if they prefer to live with little comfort on slow ships. They don't have powerful weaponry or defenses or much to steal from. They are, simply put, not worth it.
Their looks are not helped by their mistrust of everybody else. They are insular to point of being paranoid and steer away from any permanent attachment to others. They are a hard, pessimistic people, talking little and revealing less, always keeping a low profile.
The rare friendly races that would offer them treatment will be avoided by the Selan like the plague, some think it has to do with religion.
Sometimes they adopt children of other races with conditions similar to their own, in one of the few signs of charity they practice.
The Selan come from the depths of Borg space. The scourge of so many species is efficient and in its search for drones sometimes rejects those that would be uneconomical to use. Having major physical deformities or rare diseases that are hard to cure is one criteria, too deficient or incompatible technology is another.
The Borg Collective has been growing for thousands of years. Through an adverse selection process, the trickle of escapees has formed a race of their own.
The Selan make themselves intentionally hard to assimilate by the Borg, by having 'uninteresting' technology, and changing their biological specifics contrary to the 'preferences' of the Collective. It doesn't always work, it's but better to live in uncertainty than to be drones.
The Selan have lost any hope of defeating the Borg long ago and lost every homeworld. A few groups have regrouped and fought back and have been defeated, the last dozen generations have not even tried. Now they just try to stay ahead of the storm, always on the move.
Whoever they meet may not be alive next time, or be turned into a drone. It's better not to care too much.
Although they know a lot, they will certainly not help anyone to defeat the Borg. They have seen the attempts fail too many times and the Collective might come after them. If you want their help your best option is to steal their data.
Their technology is primitive to the Collective, but they have a few surprises. Warp capable, they can follow in the wake of faster ships (or cubes), achieving speeds far above what their engines allow. They also pick clean the rejects of the Borg - from simple trash to the remains of huge battles. Not a few of them have always an easy means of suicide at hand. Their vast databases of technology are easily destroyed as well.
Those who attack them might find their ships tagged by rare types of radiation that are easy to detect across large distances... and sometimes attracts the interest of the Borg.