I'm just needing to know what category, as I build another new character, do the summon spells you learn in the soul cairn fall under in the conjuration skill tree. Do they benefit from the zombie perk tree or the atronach? I'm guessing the zombie side since they're all some type of skeleton....

Conjuration. All of them do.

Heh, I know they fall under conjuration, but not which section of the perk tree they actually benefit from...lol. I'm guessing the zombie perks affect these spells....

It appears they use the necromany perk, not the atromancy. I am not sure about this, but here is the test I ran:

I have both the atromancy and the dual souls perks, so I summoned both a Wrathman and a Dremora. The Dremora lasted 60 seconds longer than the Wrathman. I do not know what would have happened if I had the necromany perk. But I do know that the atromancy perk does not effect the Soul Cairn summons.

The skeletal summons (wrathmen etc) appear to be effected by atromancy rather than necromancy. I also do not think that Arvak gains you XP in Conjuration - though the magicka cost is effected by your Conjuration skill.