needed to make a discussion post to post this all without making a thousand comments, sorry

Short version:
Overall, in terms of my (relatively informed) opinion (3 raids over 2 colonies) of which need a combat nerf,
8/10 Green Slime (DoT, leap attack)
7/10 Lava Slime (DoT, especially on animals and downed pawns (consider that this is mostly one-sided against the player), minor nerf to explosive damage on jump)
6/10 Sludge (DoT, especially the selection of the body parts when engulfed)
10/10 Green Slime + Lava Slime combo in raids (Green downs easily, Lava ignites downed pawns, easy death or serious injury)
7/10 All (Lack of pain/bleed/critical weakness making downing very difficult without complete dismemberment, and rendering capture potential very unlikely)
I don't suggest removing ANY of the above, but merely a small nerf to each. I like the uniqueness! The vast majority of the problems are where the faction's combat strategy interacts with combat animals and the disproportionate power of the abilities when used against a defending player.
Also, maybe a friendly slime faction, disabled by default?

Long version:
I also want to put in that the acid damage is really strong, especially with the DoT extra effect (the "covered" status). Not sure how much you can/are willing to change things, but, just as an example, ~10 of my colony's fighting animals (Vanilla toads, 1 Megaspider, 1 Speleopede, 1 Erin's Chocobos, all adult) were downed in a battle against ~15 slimes. Around 3 were downed or killed before being able to do much of anything, around 7 died in total due to the additional fire burn damage or acid DoT. The speleopede and one toad were engulfed, and there wasn't a way to kill the sludge even remotely fast enough to free either (he was the last one standing, was fleeing, and took at least 15 seconds of sustained fire from 7 colonists with LMGs (and decent-to-great shooting skill). This was after, at least, 30 seconds of combat shooting the other slimes while they were standing in the open, punching my toads. *to be clear, not using Combat Extended or any other similar mods to change combat mechanics

I've been playing with this mod for a while (and I do overall like it), and from my experiences, the "jump" abilities should probably either have a longer cooldown (an hour? Consider that vanilla Yattakin's beast calling is a quadrum by default and has roughly the same "average" value, if not the cost of the slime resource or the danger to the pawn if used in battle), or a minimum range of activation to prevent re-use in combat. The firefoam slimes aren't as big of a deal. But, the lava slime's ability to ignite pawns should also be considered as pawns don't put themselves out normally, instead running around like dumbasses. In addition, downed pawns (animal or person) cannot put themselves out, and frequently can't be put out by other colonists while in combat. Perhaps a lower-damage explosion, but more fire likelyhood? Or vice-versa. Are they meant to damage or ""distract"" by forcing pawns to be useless/out of position?I'd also reduce the DoT of the acid damage by ~1/2. Consider that burns tend to be very painful, so the acid can easily down a colonist without any other input. I'd also reduce the damage from the engulfing attack, or have it specifically target non-vital areas first (both deaths were from body HP kills, with no other destroyed limbs, if that helps). It already removes a pawn from combat entirely, and can be used multiple times in the same battle. If it happens on an animal, who doesn't appear to fight back much if at all (and has a lower DPS than most melee colonists, and doesn't have any armor, reminder), it practically guarantees a death, especially considering that the sludge tends to be a low priority target compared to the greens, which can close the distance and down colonists in a single attack (plus some time), or the lavas, who can do similar.

Finally, I'd reintroduce the idea of a pain threshold for downing slimes, even if it is much lower than other pawns, like 50 or 25%, since the only other way seems to be completely destroying the body, or getting lucky and striking the core, since they don't bleed out or have weak points otherwise.

All of this being said, I don't think you should remove the individuality that the slimes have as a different "species". Their unique benefits in combat are a good thing for gameplay variety, but as Rimworld is balanced around very different combatant types than them, I feel it's important to determine how to interact, and how much, with vanilla systems (bleeding, pain, fire, downed pawns, typical defense strategies like range and traps) should be considered especially. How much does a player have to change their strategies to combat slimes? Is that much change realistically doable before getting raided, or cost-efficient enough to consider when only one faction would need the benefits? Are the mechanics used fair against every target type (player vs non-player, different tech levels, etc), or how can those mechanics be changed, or new mechanics added, to offset?

Thanks for coming to my TED talk. I really like this mod and what it tries to do, so I'm invested in seeing its potential really shine through.