According to Seven of Nine, there's a simple reason for this; the Borg encountered a Kazon colony, but "their biological and technological distinctiveness was unremarkable." It seems the Borg considered the Kazon to be "unworthy of assimilation." In their view, the Kazon would not add anything to the Borg Collective; ...
Species 8472: Also known as the Undine, they were a highly advanced and biologically unique species from fluidic space. Due to their vastly different biology, the Borg were unable to assimilate or adapt to them.
Reputation aside, there is little in the Ferengi for the Borg to wish to assimilate. Physiologically, they are fragile creatures and do not have traits that the Borg are interested in. Aside from a passing interest in their auditory capabilities, it's doubtful the Borg would assimilate the Ferengi.
Tholians need a temperature of a whopping 400 degrees Kelvin to stay alive. Without it, their silicon-based "skin" cracks and splits, resulting in a painful death. This creates a unique problem for the Borg. With a newly assimilated Tholian, they would need to home them.
Could "Q" or a "Founder" be assimilated by the Borg? Yeah, maybe. There seem to be some species like species 8472 though, that are just incompatible with assimilation, but for the most part they can usually find a way to assimilate many species.
Q seemed frightened of Guinan, telling Picard that she is "not what she appears to be," and that Picard should remove her from the Enterprise. Considering Q's abilities, it's unclear why he would be afraid of Guinan, and Star Trek has never offered a real explanation for Q's reaction.
And since the Borg did not know about Q, for them, it might have appeared as an untamed tease, baiting them into the alpha quadrant. At the end of the day, whether Q's actions helped or hindered the Federation are almost second to what his true intentions were. If he helped, it's unlikely that he did it on purpose.
The Borg learn everything they know about an enemy through assimilating beings. The Borg couldn't assimilate Species 8472 because their immune systems were hyper-advanced.
Officially, the existence of a Borg homeworld is a hypothetical one as it has yet to be revealed in a canon Star Trek source. However, an identity for the Borg homeworld is given by the non-canon novel Lost Souls, which names it Arehaz.
As far as I know, the "Bad Borg" hail from the Delta Quadrant, and though we don't know just how old that collective is, it is ancient. The "Good Borg", Jurati's Borg, were established in the Alpha Quadrant in the 21st century, when the queen from the alternate timeline and Jurati merged.
Species 149 is the Borg designation for this species. Its actual name is unknown. This medically-advanced species was assimilated by the Borg some years before 2374. This species had developed technology that allowed them to revive a deceased person as long as 72 hours (3 Earth days) after brain-death.
The Borg quickly realize that Species 8472 is immune to assimilation and that contemporary Borg technology is no match for it. In fact, 8472 is one of the few species so advanced that its ships are able to destroy Borg cubes in seconds.
Locating the Queen's ship above Jupiter, Picard confronts her, and the Borg Queen explains that following the events of "Endgame," the Borg were decimated to the point of near destruction by the future Janeway's pathogen, leaving only the Borg Queen, a handful of drones and a single Borg Cube by the time that Picard ...
The society of species 1 consisted of female drones and a queen for each colony. There were males, but they were only used for mating, much like ants on earth. Through millions of years, species 1 developed a collective consciousness on the colony level, with each colony having its own.
Admiral Janeway allows herself to be captured by the Borg Queen, so that she can implant a neurological pathogen into the Collective. The pathogen cripples the Collective, destroying the Queen's ship, and allowing the USS Voyager to ride through the Borg's transwarp corridor back to Earth.
Although any further details would be purely speculative, it fits with the established canon and timeline that the ancient Romulans could've started experimenting with artificial life forms, accidentally created the Borg and realized the terror their experiments had given rise to.
Whilst the Borg are primarily a nomadic space-bearing race they do however occupy planets with a typical population of nine billion drones primarily for the purposes of strip-mining the planets of all natural resources.
However Geordi said that Data was able to move faster than Borg drones, who couldn't move any faster than a human; while Data could move fairly quickly over almost any terrain (but he couldn't swim at that time).
The A.I. at work in Star Trek: Discovery has employed similar methods, has a similar motive, and even shares style points with the Borg. It's a technology-based entity that despises all organic life and seeks to have the galaxy all to itself (and it's also currently thwarted by humans).
The major sects are Kazon-Halik, Kazon-Ogla, Kazon-Oglamar, Kazon-Relora, Kazon-Nistrim, Kazon-Mostral, Kazon-Hobii, and Kazon-Pommar. The two most powerful sects were the Ogla and Relora, who possess most of the Kazons' manpower and ships.
At that time, writer and de facto showrunner Maurice Hurley intended for the Borg to be introduced much earlier, and possibly as an insect race. Hurley was also the writer of "Q Who?" and the person who, basically, created the Borg.
While Q's powers are almost inconceivably vast, he does have to answer to the Q Continuum, and has had his powers revoked by them on more than one occasion for indulging in personal vendettas and petty squabbles. That said, there are few Star Trek characters more powerful than Q.
