The Giant Slaughterfish, although it uses the same model, is more different from the normal slaughterfish than the small slaughterfish variant.
No it's not. Unless the scale difference is really massive, you don't even notice it.
I don't even know why they bothered to make another model for a small slaughterfish.
For variety.
And the Rumare Slaugherfishes' scales are different from normal slaughterfish scales.
A scale difference is meaningless. The different scales is just a gameplay thingie for that quest. The Rumare Slaughterfish disappear once you've gathered 12 scales...
I can do the same thing in my Giraffe mod. I can give them each a different type a giraffe meat. Easy-peasy. They'll all be exactly the same as far as their attribute, model, icon, effects, etc. are concerned but they'll be
different. Bleeergh.