That seems perfectly doable!
If you haven't already (although I shouldn't doubt that you have!), you should take a look at the wiki to see which fish spawn in which biomes: https://minecraft.gamepedia.com/Biome#Aquatic_biomes
If you want to squiggle in some tendrils of cold oceans along with the big swath of warm, just reference the aforementioned linked color codes. Also, I haven't checked that file yet, but just to be safe: make sure any of your drawing to the biomes is done in an image editor with anti-aliasing OFF
. The colors need to be exact, and the blending of tools like Photoshop's "paintbrush" would introduce a bunch of invalid color codes.
FWIW, I still haven't actually uploaded any of my modified region files to the server yet. I continue running through a bunch of paranoid testing locally for now. I suspect that by the time this weekend rolls around I should have worked up enough confidence to go big.