Does the Archaic knowledge mastery affect ALL abilities that deal MAGIC damage?Such as Singed's fling and poison gas?Or only "traditional" spells?

Thanks

I believe the magic penetration will apply for all abilities that are affected by the targets Magic Resistance.

singe's fling and poison gas are considered traditional spells. so yes, it does affect it. this includes teemo shrooms.

I've always been used this rule of thumb:
Any ability that is affected by AP is affected Magic Resistance reduction.