I had a few thoughts about these fellows over the weekend.
First, do we want to have them wield two greatswords among four arms, leaving the other two for other things? Or follow the marilith example of one one-hander per arm?
How about this ability?
Localized Omniscience (Su): Seraphim continuously use arcane sight, detect thoughts, discern lies, and true seeing as the spells (caster level 20th), except all function to a range of 120 feet.
Summarizing one of the other abilities:
Holy Power (Su): Any melee weapon wielded by a seraphim is trated as a holy power weapon. When a weapon of holy power strikes an evil target, this power erupts forth and deals +3d6 points of bonus holy (good) damage to the target, and the target gains one negative level (Fortitude DC X to remove 24 hours later). On a successful critical hit it instead deals +6d6 points of holy (good) damage and bestows two negative levels (or +9d6 and three negative levels if the critical multiplier is x3, or +12d6 and four negative levels if the critical multiplier is x4). The save DC is Charisma-based.
Here is the master of magic ability we gave Monkey...
Master of Magic (Sp): At will, Monkey may use any arcane or divine spell from any spell list as a spell-like ability, with the following exceptions: limited wish, miracle, wish, spells that allow planar travel, and conjuration spells with the calling or summoning descriptors. The saving throw DC for such abilities is 17 + spell level (10 + spell level + Monkey's Charisma bonus).