"This means that the system in question could have planets with small orbits (like the earth's for example) orbiting that system's centre of mass despite the st

"This means that the system in question could have planets with small orbits (like the earth's for example) orbiting that system's centre of mass despite the stars being many AUs away from it. It may be unlikely but it's certainly possible, right?" No, this does not mean that at all! An orbit asks for a centripetal force. In heliocentric systems this force is the gravity of the star (considering the gravity of the orbiting body to be negligible). Here, pairs of stars are kept into orbits and for each star the force that keeps it around is not only the force of gravity of the other star, but also that of its own (since it's not negligible any more). A third body can not have a stable motion around the "centre of mass" between those stars because there does not exist any force to keep it there. Any body between the orbiting stars will be under forces that will pull it towards the stars. It may at most stay "stable" in the middle, where the gravity forces of the around-orbiting stars cancel each other.