User blog comment:Supahbadmarine/Official Canon Policy/@comment-4608429-20130220004829

1. Space Wolves is no no, Salamanders is a yes, as long as there not 2nd founding (as they have no recorded 2nd founding successors).

2. No, not really.

3. I believe that they can be quite big. The Galaxy is a massive place, and a few hundred worlds isn't really that much in the big scheme of things. I'd say an upwards limit of ~150 worlds is about it.

4. I think that the problem here is that people focus technology rather than the actual mindset/culture of the race, and so it can appear that they have no flaws. The way to give a race flaws is to give them a "blind spot" in the way they think, rather than limiting technology (though it does help).

Note: I think anything can be justified, as long as it has a good enough reason.