Alright, I ended up making snails rather unhappy by modifying the files
moods in00.txt and
moods out00.txt (I actually don't know what the difference between these two is) in
missions\00. These files define most of the influences to your station visitors, defined in many tables. E.g. there is a table
MOOD_AT_BIRTH, defining which species starts with which values. I changed the
(gem slug) line to start with a 5000 instead of 10000, making gem slugs half as happy from the start. I also made sure that they are a bit more picky about encountering salt hogs (
MCWP_SALT_HOG) and don't enjoy anything but their "personal" buildings (
MOOD_INC_SLUG_BAR and I think their monument is somewhere too). Modify this to your desires, I just made the snails even more aristocratic while still allowing them to achieve happiness if I really want them to.
build_changers.txt, there are two quite suggestive Variables,
the exact behaviour of which I couldn't decipher. But I guess by using correct values here, one could leave the slugs happy while having them not pooping too much...