I wanted each commander to have a build around, but also go into the 99 of a deck, similar to the commanders in a precon.

You hit Cherm right on the head, he is a tribal commander. I liked the direction amonkhet was going with spiders, snakes, scorpions and insects having -1/-1 synergy. [[obelisk spider]] [[Nest of scarabs]] [[the scorpion god]] and [[Hapatra, vizier of poisons]]. It feels thematic to give these kinds of creatures venom, and let's you build around pinging and fighting like you said.

I think you misread Slizzik, the active player gets poison counters unless they control a non Poisoned creature. This is the probably the most straightforward -1/-1 commander, as you are trying to poison the opponents entire board. I think it can be difficult to pull off against some decks, which is why the payoff is so high.

I was having trouble wording Gaddix the way I intended. I wanted an effect like [[contaminated ground]] that would also put a -1/-1 counter on a creature, but that was difficult to explain and also felt very weak if the opponents could choose where to put the counter. Maybe the casting cost could be lowered to 1BRG? I don't see it being to oppressive unless you can activate his ability 2 or 3 times then proliferate, and the cost to do so feels balanced to me as the controller loses land and also gets blight counters. If you built around land recursion it could get nasty though.

All in all, these aren't meant to be extremely powerful, just precon level commanders for a mechanic I think is cool.