Slimy Slugs will also appear in Rock Hard, if you have more than 8 miners.
They would also appear in Lava Laughter if there were any holes on the map, but there aren't.
But yes, slimy slugs are notably absent from many of the levels. According to Karl, who made many of the levels, somewhere in this thread he mentioned that slugs were only finished at a late stage of development, and would have liked them to be in many earlier levels - often the cutscenes show slugs when no slugs are present!
So the absence of slugs we can generally put down to there just not being enough time to really put them in properly.
Additionally, on a technical note, adding Slimy Slug Holes to a map is very straightforward - they're just defined in Dugg.map, and with Map Creator doing so is exceedingly easy. However actually getting slugs to pop out of their holes is a different beast.
The game has an unused, inbuilt slug script you can see in the .cfg in one of the testlevels, which causes slugs to come out at a specified frequency once some conditions are met. However, the usual way slugs are specified in the LRR levels is by NERPs scripts, which are awkward, unwieldy, and an absolute pain to write and if that wasn't all bad enough DDI's coding is atrocious.
Usually (Rock Hard is the exception) the format is something like this:
GetCrystalsCurrentlyStored > 9 ? SetR1 1
GetCrystalsCurrentlyStored > 12 ? SetR1 2
GetCrystalsCurrentlyStored > 15 ? SetR1 3
GetCrystalsCurrentlyStored > 20 ? SetR1 4
GetSlugsOnLevel < GetR1 ? GenerateSlug
which should hopefully be human readable: essentially, as you gain more crystals, this value R1 goes up, and if at any point in time you have fewer slugs than this magical variable R1, another slug pops out. This is why, in Back to Basics, killing one slug immediately causes another one to rise to take its place, and is generally super un-fun.
The simplest slug I've found is
GetMonstersOnLevel > GetSlugsOnLevel ? GenerateSlug
which makes sure there are X slugs for X Rock/Ice/Lava Monsters, including sleeping ones. This means slugs pop out as a bonus challenge, but once all the monsters go away, the slugs do too. If you have a cool idea for slug scripts let me know, as it's still something I'm thinking about (especially with Manic Miners on the horizon )
... anyway that's getting mighty off-topic of me. But yes, slugs just aren't there in most levels - and I'm so glad they aren't because can you imagine bad raider AI with erosion, slugs, and drained crystals? I don't even want to think about it While Rocky Horror having no slugs is a bit of a shame for the Final Boss Level, it means it's actually fun instead