the Jester said:But the description of hold effects is different from the description of paralysis effects, iirc. Also, hold effects are mind-affecting, while paralysis (typically, anyhow- there might be exceptions) aren't. The two effects are qualitatively different.
I think you may have misunderstood me somewhat. I actually agree that Hold and Paralysis are two different things. No, dragons should not be immune to Hold spells, IMO.
But even if there were more "cerebral" Paralysis effects, I think the dragon would still be immune, because it is still a Paralysis effect. Whereas, to me anyway, a poison that paralyzes is doing so as a Poison effect, where being paralyzed is the consequence for failing the Fort save vs. said poison. Unlike a Paralysis effect, where you are being hit with it as a Paralysis effect and failing your save makes you paralyzed. The dragon still has blood in its veins that can carry the poison and [insert how certain poisons affect the body here].
Plus, any attack that has a Paralysis effect would be magical in nature (Sp,or Su). Whereas, poisons that can paralyze are Extraordinary (non-magical).
Does any of that make any sense at all?
the Jester said:I'd also say that something immune to poison would be immune to any poison, regardless of its effect- whether it inflicts paralysis, blindness, strength damage, con damage or whatever.
I agree here too. Immunity to poison would apply to all types of poison. Even using your criteria of something having to be in it's own catagory.

the Jester said:Also not trying to be argumentative- just trying to explain my reasoning! And I could be wrong- idhmbifom.
I also dhmbifom. So we are even.
