How about solitary, cloud (2-4), or nebula (5-20) for the swarm org?
Sure, I'll go along with that assuming Arnwyn agrees.
I do indeed agree (I like the variability as well and prefer dice rolling to just a low static number (1!).
I also like "Brood" (just because it sounds more menacing, and my players react more to that kind of stuff). "Jive" would be... hilarious (though, I admit, it is Spelljammer...).
CR 4, then maybe? The taking horrible damage is a bigger factor than flying (which is pretty common in Spelljammer and everyone is prepared for it).
I lean toward a weak CR 5, too, but it really is on the edge.