If I'm on my Zon and I use plague/poison javelin, then shoot guided arrows with a poison small charm equipped. Does that poison small charm dmg override the poison/plague skill?

I tested it out in game with my poison necro with a similar scenario. First I cast poison explosion 102k damage over 20 secs. Then I cast poison nova immediately 12k damage over 2 secs. The monsters stayed poisoned well over 2 seconds. So I conclude that the higher poison damage attack overrides the lower poison damage attack no matter if it landed first or second. I think in your case the plague javelin would override the psn dmg small charm.

Poison damage from multiple attacks doesn't sum: if a target is already poisoned, then the rate and length of that poison will be overwritten by that of any subsequent attack which has an equal or greater rate. Rates are compared after all modifiers have been applied.

https://www.theamazonbasin.com/wiki/index.php/Poison_Damage

Can anyone else confirm?

If it reads in amazonbasin it is 99,9% true.

Yes, higest dps overwrite.

so no matter which one hits first, the higher dps poison source will always remain, correct?