Toxic weapons only work on a damaging hit (ie one that penetrates armour and Toughness).

Creatures with the Toxic trait have their poison work after merely hitting the target (ie a succesful WS test). No need to penetrate armour or Toughness. Is this how it works or did I miss an Errata round?

I honestly have absolutely no clue, but no matter what the RAW is, I'd argue that anything Toxic actually has to do damage to apply, unless it happens through inhalation for some reason.

I think so too. If you take the Slaugth for example, they just have to hit, not even wound, for a straight 1d10 damage ignoring T and AV,

It can be acid, flesh eating bacteria, other corrosive fluids and gases. Therefore no need to have flesh wound in order to take effets.

I think it was stated in toxic quality in core book that each damage you made through with the needle itself, target gains -5 to the toughness check to resist toxic.

Ie you do 3 damage through and the target gets -15 to resist toxin damage.

Toxic quality in Rogue Trader:

"Anyone that takes Damage from a Toxic weapon, after reduction from Armour anbd Toughness...."

So, you need to take damage from the original attack before Toxic damage applies. That's why Plaguebearers are such a pain in the ass with their plagueswords. My Exploration took a single wound from a plaguebearer attack, faliled the Toughness roll and blam! +9 extra wounds.

TOXIC
Some weapons rely on toxins and poisons to do their damage.
Anyone that takes Damage from a Toxic weapon, after reduction
for Armour and Toughness Bonus must make a Toughness Test
with a –5 penalty for every point of Damage taken. Success
indicates there is no further effect from the weapon. Failure
however deals an immediate 1d10 points of Impact Damage to
the target with no reduction from Armour or Toughness Bonus.

this is how Dark heresy rules it.

It's the same wording RT uses. I don't see any problem with the rules then, you have to actually deal damage with the original attack before the Toxic quality applies.

So, for example, if you have a poisoned knife (1d5+SB) and your enemy has Storm Trooper Carapace (6 AP), you probably won't do him any damage, which means the Toxic quality is of no use.