I think I'm going to leave it as a non-tanar'ri, mainly because they were created by a marilith. Also, they struck me as serving a role similar to the yochlol, which also (at this time, at least) do not have the tanar'ri subtype.
Moving on, here's another resident of the marilith Kaliste's domain.
From the Pools of Darkness Adventure Journal:
Electric Spider
AC:4 HD:5 Mv:12 Int: Low Size: L
Align: CE THAC0/:15 Att:1 Dmg:2d4
Spec Att: Lightning breath
Spec Def: Immunity to electricity XP:650
Electric spiders can be found throughout Kalistes dimension. They can
breathe a lightning bolt similar to that of a blue dragon's. They
favor narrow confines where they can bounce their bolt through a
victim several times.
I'm thinking Magical Beast (Extraplanar) for these guys. Any thoughts?