Strictly speaking unless a creature is stated as having an immunity to a particular ability they are affected by it as normal.
As to your specific examples:
A Ghoul is an Undead creature, and as such is immune to paralysis.
Spiders are not immune to the bites of other spiders, and that's true in the real world as well.
Undead are immune the paralyzed condition. The poison thing...um, is a little more sticky; some monster are immune to their own attacks but spiders aren't immune (to my knowledge) to poison their own or other wise. They can move over webs though.
Elemental creatures are immune to some of thir own abilities because of immunities or resistances so Mr. Fishy would suggest check that first.
That said DM's call.