They were extremely shy of other species but with a little coaxing seemed to do fine in interacting with other species as soon as they were sure they wouldn't be harmed by them.
They did not possess any verbal skills and it was indicated they were physically unable to speak. This may have been due to the lack of vocal cords or similar organ.
They did seem to possess a language of their own, because when The Doctor taught Fantome to communicate using a system of musical notes, Fantome was in turn capable of teaching this language to other members of his species in a fraction of the time it took him to learn. Seven of Nine, who had first conceived of the idea to communicate with them in this fashion, noted that the language seemed to be developing its own grammar and syntax which suggested that they did have their own already existing language. The Doctor theorized it might be telepathic but he was not sure about this. They seemed just as comfortable communicating with music after learning how.
They were an extremely intelligent species. They were capable of learning new things very fast and adapting to benefit the most from any circumstances they found themselves in.
They also possessed a unique and extremely complex physiology. They possessed a very large lung capacity, which allowed them to survive in an atmosphere devoid of oxygen for a certain period of time. They also seemed to have a voracious appetite, this was probably so they could eat as much as possible to build up reserves whenever they found food.
They were a nomadic people, but they did have a sort of 'homeworld'. They refused an offer to be taken out of the Void, even though this would have given them a far easier life. They considered the anomaly their home and simply requested to be beamed to a random ship in the Void. They did however show the crew of Voyager their gratitude for being welcomed on board and being treated with kindness by aiding the ship and its small group of allied ships to escape the Void by sabotaging the engines of their opponents. (VOY: "The Void")