Your spawn rules currently support only two categories: "hostile" and "passive". Minecraft actually has four mob categories: "hostile", "passive", "ambient", and "water".

Clumping "ambient" and "water" in together with "passive" is annoying, as any time I want to make spawn changes to passive mobs I have to then turn around and revert those changes for ambient and water mobs on a per-mob basis. This is incredibly tedious in modpacks which add several ambient and water mobs, and gets very tricky when trying to make seperate rules for passive water mobs and hostile water mobs.

As a use case I would like to be able to apply the following snippets:

{
   "passive": true,
   "water": true,
   ⋮
   "result": "allow"
},
{
   "hostile": true,
   "water": true,
   ⋮
   "result": "deny"
}

For example in vanilla minecraft this would allow squid to spawn, but not guardians, without altering any other mobs (except possibly the Elder Guardian? Does this mean we need a "boss" category too?).

I realize this is an old post, but you could just do this to stop the guardians from spawning while keeping everything else

{
"mob": "minecraft:guardian",
"result": "deny",
}

-note I am not the mod creator, just trying to help