Well, for grabbing all participants, that was in case you wanted some enemies to be naturally already resistant to the damage. If you're not going to have something like that, that's less work to do.
Basically the grabbing all participants was so that you wouldn't need to check to see if X member was in the party before checking everything, you would instead check, say party member 1, no matter WHO that was. Same for enemy battlers. Without plugins, you'd only have to check a max of 4 party members and 8 enemies. If you're handy with JS you could even code a loop for that.
As for the plugin... if I'm seeing it right, it wants a skill to add a state, which will then point to another skill for the actual damage. So you'd essentially need two venom skills. One to apply the poison state, the other that will actually do the damage. So the first one would be the one the enemy actually uses, and all it will do is apply the state (unless you want any other immediate damage dealt by the skill), and then check to see if any battler has the resistance skill and add that state too. The second is one that no battler actually knows, and it's damage box would have your calculation code. You'd have to compress it into a single line, but it should be doable.
Then in the common event triggered by the second skill(I'd suggest checking to see if the plugin fully uses the second skill rather than just borrowing its damage. Maybe stick a message in the common event that only gets seen if it works the way we're assuming), check all members of the party for the state (the ones that can learn the resistance, anyway), and increment a variable tied to them. Then when the count hits your threshold, have that actor learn the resistance skill. Probably achieve this by setting a switch for the actors or just keep checking that the tick variable is above the proper threshold.
You may have to tweak it a bit to make sure that the resistance state only triggers once. It's possible that it also won't increment the resistance variables properly on turns where the poison state wears off. I'm not sure how that stuff is timed.
I hope I'm reading that right. Theo's coding style is remarkably compact, but that also makes it a little difficult to legibly parse. And I'm not all THAT familiar with those parts of the engine's combat code anyway.
Of course you could always attempt to code your own plugin, but that depends on your skill with JS and your familiarity with how the MZ engine works.
Kinda rambly, I hope I made ANY sort of sense. I've been out of the game a bit and haven't tested ANYTHING I've told you, since I've never needed to try to do this kind of thing.
Obviously this is easiest if only one character in the entire game can learn this. The more characters that can learn it, the more complex it might become, depending on various factors in your gameplan.