I don't have a solution to the issues with this tag, but I think that the real problem is that it's simply a very niche and very obscure tag. Few people know it exist, and when it's used by someone outside of the usual suspects it's often used canonically. Stuff like post #5006878 should not be tagged interspecies, but coom-brained taggers don't bother to check.
Monster girls are a case-by-case basis, and as blindVigil said you shouldn't tag a random kemonomimi broad with interspecies, but consider this example: even for something as alien as slime girls, almost nobody uses interspecies. Compare slime_girl interspecies with slime_girl 1boy rating:e -interspecies.
I think creating furry with non-furry and implicating it to interspecies has helped its usage a bit, but I don't think we should create more granular versions. I expect that if we did that people would just not use them like they already not use interspecies. It's one thing to do it for furry, it's another to do it with random subgenres that at best would be gardened once a month by a single user.
That said, I think it's fine to use the tag on sufficiently inhuman characters like robots (think darling in the franxx, not "I'm actually a 3000 years old android") or lamias/centaurs, but the issue is that certain users are just going to be pedantic and tag those canonically if we spell it out in the wiki. Does someone searching for interspecies want to see a Centorea paizuri where the horse parts are not visible (post #3534591, currently tagged interspecies)?
I love post #113417 for briging a very clear differenciation between all these humans and non-humans. Based on this, I would consider "Non-Monster Girl Type C and D" what would fall more into non-human in this matter, and thefore interspecies. I don't see much use in tagging all monster girls as interspecies as 99% of the questionable/explicit posts would fall into that. What really cames to mind with interspecies are furries, aliens, and humanoid creatures, things like that. Virtually all monster girls are already paired with humans by default. Of course I don't mean this applies to literally all monster girls, because some of them look more monstruous (like "Non-Monster Girl Type D") or fall into both categories of monster girls and furries.
In this case it doesn't really matter if something is the default. ~monster_girl ~interspecies is not an acceptable solution (just look at the results), so it has to be tagged on monster girl posts where the girls are sufficiently inhuman, otherwise you might as well just nuke the tag, because you'd be removing a large portion of it for no reason.