missnanami99 Since the OP hasn't logged in for a couple of weeks, I took
a few minutes to check this out myself, and it
does work on mermaids! Of course, as I'm sure you already know, it also works on non-mermaids, which might not be ideal. But it absolutely is a pack-independent method of doing what the OP asked for, which is impressive work, especially for a new modder.
If the OP turns back up and is interested, though, I'm still down to see if I can use the mermaid-specific tuning from the EP to make a mod that only affects mermaids. It's obviously not a failing on the part of a modder to not have an EP, but while I think it might actually be possible to
make a mermaid-only mod without having access to EP-specific tuning (though not with the method I plan to try), it's impossible to
test it without the EP, so I'm glad to see what I can do. (Not sure if I can do any better than just making it only apply to mermaids, though. You might still have to make sure they don't have other motives turn red in the tub.)
Also, OP, you may find the 'mermaidmotivetweak' download
here interesting. It's only really useful for mermaids who can hang out in the ocean all the time, and I'm guessing you probably had mermaids who
don't live on ocean lots in mind when you asked about this mod, but it still seemed relevant to your interests.