Actually I would argue otherwise. Your troupe's assumption might be valid for cases where a magus was knowingly drinking a magic potion of his/her own volition, but "disguised in a glass of wine" means that the intended victim does not know of or wish to experience the intended effect and thus would not have lowered his/her Parma.
There is no ambiguity in the rules on this that I can see.
Yep, this is my point of view, and the RAW are not ambiguous at all on this point.
But I think my SG wants to build an atmosphere of "cloak and dagger" or something like that...
We/I do like house rules, (when necessary,) you have to understand that 
Our gaming group doesn't always agree on every rules, the SG decide on those cases. We try to solve such things after or before the game (such as now: SG, where are you ? come explain yourself. eh...^^ )
House rules are fine where there is some element of ambiguity perhaps or where the House rule might improve the capabilities of the covenant in some way, but I would consider such a house rule about Parma and unwittingly imbibed potions to be completely antithetical to the clear established rules and thus detrimental both to the balance of the game and the raison d'etre of Parma itself.
There are plenty of other ways to insinuate cloak and dagger without completely undermining the entire value of Parma (as this house rule does). Under such a rule, one clever magus could systematically murder every other member of the Order if he/she so chose just by travelling around poisoning covenant water/wine/etc. supplies.
"Game balance" is an arbitrary concept. That house rule might make Creo Poison unreasonably effective - but it will certainly cause the desired stylistic effect in the campaign as well. Alternate precautions will be found, paranoia increased.
Tweaking the rules away from as-written to support the campaign is not unreasonable at all. A campaign based on predestined doom, a greek tragedy or dark fairy tale could very well be most entertaining 
A clear case of IMS or YMMV then because I dont find the notion of all but useless Parma to be remotely balanced under any circumstances. Might as well never had Bonisagus invent it or the Order in the first place if killing magi can be such a dawdle.
To each their own.
Actually,. killing magi is rathere easy. The problem is killing them fast enough for the rest of the buddies not to notice and tear you appart. Killing one or 2 unaware magi up to 20 years out of apprenticeship? Sure! Any day of the week.
Xavi
Perhaps by mundane means but generally not by magical means (thus leaving traceable identifiers for the Quesitors to track down the guilty magus/magi and have them marched).
Point is that Parma is either the hallmark breakthrough of Bonisagus (as per canon creed) which affords magi the margin of safety to make the Order an attractive alternative or it isn't (as in the case above).
The actual margin of safety of Parma is negligibvle. A fairly average combat maguis will have mastered penetration with his killer spells. Parma is rendered useless against most of those spells, since its value is rarely THAT high.
What Parma really does is reduce the problems of the Gift to zero among magi, aso you can interact without decidingf the other is cheating and POFing him to death.
The real deterrent to mass killing is retribution by the other mages, not the difficulty of killing a mage. Pretty much as the real world: if you want, you can easily kill 3 or 4 random dudes in the local supermarket (inm Europe, where weapons licence are difficult to get, mind you), but it is unlikely that you walk away unpunished.
Cheers,
Xavi
IMHO the mastery Ability of Penetration is nice, but not a sure thing. I mean, Mastery Score adds to your casting total, plus it adds to Penetration Ability. But unless you somehow have a greater Penetration Modifier than 1 (arcane connection etc.), it only means adding the Mastery score twice. How many people have Mastery scores through the roof? Quite a few combat specialists migth have it in their favorite spell. But unless it's a very exotic spell, even more magi - who somehow want to safeguard themselves from such an attack - will have Mastery Ability of Resistance with that same spell. It works in my book. Parma is powerful.
I don't want a saga, where everyone is afraid of one "sure fire total kill" option, so I'd rather not House Rule in any loopholes in Peneration/Resistance. IMHO if someone slips you a magical poison, you'd resist it. Is someone gives you a beneficial potion, you'd have to have it Penetrate - or lower Parma. If you're KOed, tough luck!
This might be going way off topic.
If were still talking about whether items need more range than Personal, I'd say yes! A range personal item only affects it self (say a sword whch can become sharper, sprout flames, become invisible) - save for Talismans. Since they're a part of you, range personal can affect you, but not any one else! A range touch device bestows the effect on anyone it touches, it need only be Target:Group to affect many at once. Individual is enough to affect as many targets - one by one - as the #uses/day allows. The effect lasts untill duration dictates it ends, even if the item leaves the scene. One could claim, that a Range:Touch device needs to Penetrate, unless you suppress Parma (and a Personal need not). One could also claim, that a small item held or worn is "within" the shell of the parma. YMMV
I agree with Boxer and Ultraviolet on the Parma issue.
Moreso, if you want to poison a magus, you can still send him a mundane poison
Fixer wrote:
Yes certainly!
But you still get Form bonus to natural resistance! Since my Maga i Aquam specialist, she'd not be very bothered by mild poisons of this type. Aquam would cover (al)chemically produced poisons. While one drawn directly from a plant or animal surely must be covered by Herbam or Animal respectively. I mean, if you were bitten by the animal itself, or stung på the plant's thorns, the resistance would not be Aquam, just because of a poison-effect, right?
And about mundane poisons, or mundane items more generally: In my Ars group, we have a saying: "Idiot, a dagger always works!"
It is a reference to some fantasy novel, in which an old mage had a really magical dagger, which could do all sorts of nice things, you could call it the base of his power. He had an enemy, who attacked him, and cast a spell to destroy the magic of the dagger. He said to the old master: "You idiot, your dagger is worthless now, you have no magic!". But the old master just stabbed him, and replied: "Idiot, a dagger always works!"
loooooool
Great story 
And about the poisons, you're right, too.
So, no need to kill of parma with house rules, mages just need to think outside the magic box^^
And, a little paranoia always keeps the PCs on their toes.
You can magically ward youself against any mundane thing, if you are just prepared. But then you must cast a dozen of spells or activate a dozen of items every sunrise/sunset (or often enough to cause problems). And you might suffer Warping. Even if the effects are low and designedfor you, IMHO such excagerated magics should Warp! Look at the sum of spell levels or something.