I haven't found anything in searching that directly addresses this, so here it goes:

Assuming you have a spirit that is in posession of both a stun and physical track (it isn't a garden variety summoned goon) how is it affected by:

Normal weapons that do Stun Damage while it's materialized.

Stun Spells either Astral or Materialized

What happens when it's Stun Track fills?

Whatever you want to happen, I would imagine, since what you're dealing with is 100% houserule territory. Such a thing simply doesn't exist per the RAW. If you want official rules for a powerful, unique Spirit, then you should be looking at Ally, Great Form and Free Spirit powers rather than fundamentally changing the Spirit rules. I suppose the easiest thing for what you're warnting to do though is to simply let Spirits "lose consciousness" per normal PC rules (IE, incapacitated once the Stun monitor is exceeded) and only have them disrupted from the meat realm when their Physical track is exceeded. I guess I could imagine what it would look like if someone "knocked out" a Beast or Guardian Spirit, but if that's what you want to do I don't see why you wouldn't just make an exceptionally powerful Paracritter or NPC Awakened character.

Disrupted. Follow the normal rules for disrupted spirits.

Disrupted as soon as *either* track fills.

So, you might as well use stunbolt.

Semi-related question, but I remember not being able to figure this out from when I was reading...

Vehicles/drones specifically do not have a stun track, but do spirits have one? Or are they immune to stun damage too?

Spirits are not immune to Stun Damage.

If it has an aura, it can probably be stunned.

More Spirits have a bonus to Body than a penalty. This means, on average, that their stun damage tracks are shorter than the physical tracks.

A Materialized spirit is simply disrupted if it fills up its physical or stun condition monitor.

Possession spirits who are stunned while joined to a body are just that - stunned. They aren't disrupted unless they are actually killed through physical damage overflow. This means that it is not possible to "rage death" by having your spirit pop out from damage and have your condition track contract you to death.

That's on page 94 of Street Magic and it is a very specific and important rule.

-Frank