The latter bit is actually pretty well-hinted within the backstory of the second game and stated outright in earlier versions of the game's script. The former bit on the other hand is pure conjecture. However, since it's been known for some time know that the Homeworld galaxy is modeled after M51 (aka, the Whirpool Galaxy) and it's been stated that the Progenitors originated from a different galaxy originally, then, assuming that the Homeworld galaxy actually is M51 (rather than simply resembling it) then it isn't too much of a logical leap to assume that the Progenitors came from the Milky Way, colonized the Homeworld galaxy, and then declined to more primitive levels, becoming the Kushan (who, as of Homeworld 2, canonically look exactly like humans).