I still have the poison rules from the Dark Heresy Game Master's kit. I tend to use those as it allows the versatility of multiple effects. Although I'm not exactly sure how long the poisons are supposed to last still but that's ok.

Do you guys do the same? Modify the Toxic Quality possibly, or something else?

Well. The kit gives you a good "grasp"on how things are meant to function. And fortunately, the kit has its own examples of two poisons mixing the effects

As in regard to time, the effect (damage) occurs after the "speed" has past (immediate or after a period of minutes or hours). The damage "lasts" until "curred". Attribute damage is liste in the core rules, if I remember right it was an hour per point or something. You could of course change that to "days" or "permanent", but players HATE permanent attribute damage. Especially if it is more then one point (and if you ask me, they are RIGHT to do so. Just not fair messing a pc up like that this way.).

A diggested poison could cause internal bleeding (use bleeding rules, but no medical treatment. Well, at least not with first aid), a poison could simply induce "deep sleep" for a number of hours (exchange "point of attribute damage" for "hours of sleep).

A poison could simply blind the victim (for ours, day or permanent).

Messing up the diggestive organs so that the victims bowles will not keep food for a number of days (insteadt of point attributes damage) is nice as well. EAch day, the afflicted victim needs to pass a toughnes test or it will have a (constant) point of fatigue AND lose a number of toughness points (not regained till the poison effect is no more) due to..well... not keepingg food in.

If you want to make a character really getting edgy, use the "will blind you over the course of 1d5 hours after injection" thing and fill up a needle rifle with it. A great way to deliver the message that "one should not look".