As matter of fact, the form of the spirit has no influance on his stats.
QUOTE ("Streetmagic")
Spirit Forms and Combat
Spirits materialize in all manner of forms, including those
equipped with weapons such as swords or claws. These “weapons”
are (obviously) part of the spirit’s materialized form,
however, and do not provide any extra bonuses. A guardian
spirit who materializes with a sword in hand, for example,
does not gain any benefits of a sword’s Reach, nor does the
sword deal any more damage than the spirit’s materialized fist.
Note, however, that there is nothing that prevents a materialized
spirit from picking up an actual weapon and using it to
full effect (though most likely in an unskilled fashion).
In redard to the aid sorrcery question:
Yeah, that is a nasty one. The problem here is, that it can get out of hand fast. (But here you have to use the rule, giving a comulative -2 modifier per spirit present)
In general you need a lot of spellcasting dice, to later on crack the counterspelling.
Counterspelling beeing Willpower (easy up to 9 or even 12), Skill (6 to 9)+spec, Mentorspirit, Adeptpower, Initiation grade, helping spirits...
But just adding spirits*Force dice is to much here, too. (Two spirits force 5->10 dice->3 hits)
So the rule limiting the max. Dicepool to (Skill+attribute)*2 is reasonable.
And even with this rule, you should be thinking about limiting it to 1 "normal"spirit and only allow additional ally spirits or free spirits...