1 Is there any limitation to who can be possessed by a spirit of the possession traditions (i.e. does the vessel have to be voluntary)? If not this seems to be way overpowered. For instance, if I summon a spirit and have him try to possess an enemy mage at the back of a group of antagonists, if he succeeds, I have control over that person until sundown. Granted the spirit can't then use the mages (or any other person's skills,) but the vessel is massively powered up. Now imagine doing this with your average troll sammy. You suddenly have taken away the oppositions most powerful players and can attack from within their midsts. The ony way to rid the vessel of the occupying spirit would be to use banishing. To add a nice twist to this one what would happen if the mage had channeling as a metamagic feat?
2 can a vessel prepared by a different mage be used for your spirit? and as a corrallary can it be used if the mage who made the vessel doesn't want your spirit to occupy it? and as a further corrollary if they can use it does it make a difference if the vessel is touching the mage who made it? and will they know without a perception test?
3 is there a limit to the number of spirits that a vessel can hold at one time?
4 do bound spirits stay in their vessels constantly or do they "retire" to their metaplanes to await your orders?
5 does it require a service to have a spirit occupy a vessel?

My answers to the above questions.
1 a vessel does not have to be willing to be possessed. However, I would put a hefty dice penalty on the spirit trying to occupy an unwilling victim. As to the question about channeling, I would say that the mage retains control of the body but the spirit can use it's powers however ordered.
2 yes, I don't know x3 But you could really mess with the mage who has a predator IV as a vessel.
3 one spirit at a time. Any type of spirit .
4 they retire to the astral until called just like any other spirit (yeah it may be a stupid question, but it seems they may be differemnt since it's a different system.)
5 depends If they're occupying an unwilling victim defintely. if they have to possess a vessel to complete a service, no. If they are just being called into your body to make you tough as nails, yes.

anyhoo, I'd appreciate other people's thoughts.
btw I'm going to be running a possession based mage in an upcoming sereis of sessions and that's why I'm asking all these questions about possession based traditions.