I've heard that a Bowazon should carry one psn SC to prevent monsters from regenerating, is that true?

(My build is not using Gores for Open Wounds, which would be the more reliable source of that effect)

Poison stops regeneration. The rate of damage is immaterial.

So you confirm that carrying at least a single psn charm is recommended (and the actual psn dmg on it doesn't matter)?

If you don’t have open wounds, then yes. The length of time is more valuable than the damage itself.

Amount of damage doesn’t matter, however, if your merc has Treachery you’re goosed, because the venom damage will overwrite any poison damage you have. (Higher damage rate overwrites the lower rate and venom lasts for a very short time)

Thanks for the clarification. Another question you may also be able to answer: Does carrying a source of cold damage help with slowing monsters? I am not using Raven Frost so this natural source of cold damage is not available for me. Would a 3/20/1-2cold dmg SC do?

Each cold source adds chill /freeze time. Each charm adds 2s normal. Some uniques have longer lengths, again it adds. Raven Frost is 4s cold length. All times halved in NM, halved again in Hell.

You should have 1 point in FA for any physical bowie, and all sources will add to freeze length. Your charm will only last for 1/2s in Hell, so you want a few more sources of cold to slow things down.

With one charm giving 0.5s chill in hell and attacking with the 89ias BP for Windforce at 2.77 attacks per second, we should already guarantee 100% chill uptime?

Does the point in FA give any (hidden) passive bonuses or are you recommending it to be able to freeze monsters on demand by actively using the skill?

Yes, use it to freeze stuff. It may be situational, but as I only play HC, stuff that stops things moving is a good thing.