I was wondering how plausable it would be for a mage to play the role of a combat phyad/cybersam, using summoned possesion spirits to boost their abiliteis as opposed to cyber and bio?

Rules-wise: a lot depends on your GM's interpretation of "service." Well, ALL of it depends on that....
RP-wise: it's a crappy deal, because it's not "you" in the action. Again, your GM could even decide HE gets to play the spirit as an NPC. It's not "you" kicking the vast amounts of ass that will get kicked; it's the spirit using your body as a tool, just like you would use, say, body armor. And think about how you use that body armor. You get it shot up, then take it off and throw it away. But in Soviet Russia, body armor gets you shot up, then takes you off and throws you away. Look at what happens to your condition monitor when that F8 Guardian spirit steps out. Hope Guardian spirits don't have a penchant for the type of situation where you typically take a lot of damage...

High end game: Maybe, but you need at the very least channelling for it and you'd like to have invoking and plant spirits in your tradition to pick up regeneration and avoid the above situation.

Potentially the real cheese comes from ally spirits who don't have services but thats a whole different issue.

The rules suggest allowing the mage to play the combined entitiy even before channeling is available, as they're a backseat driver to whats going on.

Let me tell you from a GM who had such a mage in his group and who also played a possesion based char:

It is really annoying to constantly struggle about what is a service and what is not. And it really makes for some boring RP, even if you play the spirit in control. It really looks nice on paper, but I did not enjoy the bookkeeping of services and spirits available.
Let's not even speak of the expenses of high force bound spirits.

Currently playing a mage filling the heavy fire support role with a 'regular' magician, focussed on support spells and his shiny HMG. Sure, it does lack the superhuman powers moment while possesed but comes with way more utility. A question of personal preference I guess.

You'll want channeling for that character asap, then you just use your own skills all the time and get the nice stats buff from the spirit and maybe one or two powers he's constantly using on you. You'll still be able to cast spells then, so don't forget to learn spell casting as well.
Such chars can be game breaking though, depending on what the rest of your group is playing ofc.

Well can I ask you what you think is the largest action which would not be considered a service?
And what is the biggest thing which would be one service?

you can also do alot with a mage with non combat spells like orgy, elemental wall, analyze device, buff spells and invisibility, etc.

something along the lines of, after your already possessed your everyday non-combat stuff. walk over there, pick up that kitten and pet it. Anything that requires a roll is definitely going to take at least one service, though a service, if worded correctly can constitute far more than just one roll.
BTW I always found it useful to have the automaton vessels for spirits to occupy. thus you can either be yourself or the vessel, and you can have freinds in melee.
PS I started some threads in the past as to what constitutes a service, you might try your search fu.