So I've been searching around a bit and I can't seem to find a mod that does something similar to this. I'd very much like a mod that adds a bunch of smaller spiders to the game, I'm talking, about the size of a small cat. These would preferably come in swarms of sorts, so be extra freaky.

Why I'd love a mod like this is because, to be quite honest, after a certain size, spiders just stop being disgusting and creepy and become plain monsters, which is a tad boring.

I have been running Skyrim Monster Mod (SkyMoMod). This mod adds more creatures and variants to the game, some of which are smaller spiders. They dont attack in swarms, but its a great mod none the less.

http://skyrim.nexusmods.com/downloads/file.php?id=9694

The creator of this mod did an excellent job!

actually it was by finding this very mod I got the idea of the small spider mod request ^^ But yea, I've been enjoying the monster mod big time

Not quite what you're looking for based on your post, but http://skyrim.nexusmods.com/downloads/file.php?id=10175 (http://www.gamesas.com/topic/1350396-mod-in-work-for-more-realistic-animal-predator-behavior/) adds small spiders and other tiny critters to the game. They're not aggressive, and they're more tarantula size than cat size, but it's a nice touch anyway, and I do usually notice them in groups. I highly recommend the mod either way, although I don't know how it plays with Skyrim Monster Mod (considering all its AI tweaking).