Request: Hey, Does this Rag Smell Like Chloroform to You?

[Ingus]

IMO Toxic Expertise should not increase the damage, or else it would be overlap and/or outshine Toxic Power Attack.
Moreover, you should consider weather Toxic Power Attack should be applied to ammunition: you break the parallelism with Power Attack, is it intended?

This said, here my lil contribution.

Toxic Penetration
Prerequisite: Craft (poison) 15 ranks
Benefit: Whenever applying or using a poison, you may spend a full round action to enhance the toxin so it is still effective against opponents normally immune to it.
To do so, make a Craft (poison) check against the DC listed below. Failure means that the poison is applied but it will not affect the target creature. If you fail by 5 or more, you are not aware of the failure.
Success means that the poison does half his normal damage to the target (minimum damage 1). If the poison doesn't do damage, the target has a bonus of +4 to the saving throw.
Special: You have to be aware of the target's immunity and its source. Knowing the immunity allows you to "go random" and try to counter one type of immunity you state in advance. Poisons that damage an attribute cannot function even with a success if the target creature has no score in that particular attribute

DC Table:
Plants and Oozes: 25
Constructs, Elementals and Undead: 30
Extraordinary, Supernatural, Spell-Like Ability, Magic, Untyped: 35

Epic Usage:
Divine: 50
Divine, for divinities with the Poison portfolio: 80

Brewer's note:

Spoiler

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Loosely based on the Molydeus monster's ability (FC1)