First, I like your mod. It adds some variety to adventuring, and I like to have the Slime King in game.

However, the config file is woefully lacking. Here are my issues with it:

1. Not all slime types are exposed in config
   They simply need to be. So far you only expose spawn chance in the config file, and that only for some of the types. All of them need to be adjustable in config.

2. Spawn rates are way off
   On my test world, my base is on the edge of a village in a swamp biome. I get red and yellow slimes all the time, rarely a purple one, very rarely iron, but knight slimes are all over the place. This does not correspond with the values set in the config file at all. If there are biome-specific modifiers, please describe them. Ideally, you might expose those modifiers in the config file as well.

3. Haunted slimes are overpowered to high heaven
   While it's a cute idea that haunted slimes fly around the Nether being nasty, they are way too powerful. Even the small ones shoot exploding wither projectiles at players, which puts them on the same nastiness level as the Wither itself. They must have an option to control their spawning, and ideally also toggles to disable the wither and explosion effects of their attacks. Side note: the explosions are actually the worst in packs that have the Enhanced Visuals mod, since that one blurs the screen when something explodes close to the player, which means players have constant blur during attacks from haunted slimes.
   Alternatively, make them drop Nether Stars. If they are as brutal as the Wither, they might as well yield the same reward (only half-joking here).

4. The King Slime is a server killer
   I once had a King Slime spawn just when the player it spawned close to was logging off. Since the area was chunkloaded, the minion-spawn mechanic caused almost 3000 slimes to spawn around the king before the server gave up and crashed (number confirmed, this is not a guess). This needs to be capped, or worked around some other way. Again, exposing the minion spawn factor of the King Slime in config might be a good idea.

These are my thoughts after trying your mod for a while in my pack. It's a cool mod, but it does need some work in my opinion. Without those fixes, I cannot really keep it in my pack, and I would very much like to keep it.

Anybody home?
If you want this split into separate issues, I can do that. If you want me to clarify, I can do that as well.

Try my bugfix release:
https://github.com/TheCutter/Better-Slimes/releases/tag/1.0