I like to organise my CC pretty obsessively, because sometimes I change my EP configuration, so whenever I download CC I check it in-game to see if it has an EP tag, and then I add that EP name to the file name so that I know to only use that piece of CC when that EP is installed.

However, I'm realising that the EP tag system might be incorrect, as I recently installed some CC that I knew required Pets, because it was pet-specific, but in-game it was tagged for Late Night. So now I'm wondering how many other tags are incorrect.

Is there some more accurate way to see what EP a piece of custom content requires so that I can label things correctly and avoid potential crashes and lagging? I'm comfortable looking at SimPE files, so feel free to direct me there.

Not sure what you mean by "tagged"? The symbol at bottom corner in CAS??? CC can be cloned from whatever the creator wishes. I personally clone everything from basegame donor, or convert group ID to basegame, except Plumbot parts which require EP11, so I clone from EP11 bot parts. If scripted to specific function, that can be altered in s3pe. I believe a script only works if you have that EP installed.

If it bothers you a lot, you can make a new package using TSRW and renumber the group ID to 48000000 (Pets). Doing that in S3PE usually does not work, as TSRW rewrites all references of other files to the new ID.