Couldn't you add some LUA in lua\server\EMoreFish_recipecode.lua like this:

function Recipe.GetItemTypes.Fish(scriptItems)
-- Get YOUR items with Tags = Fish
scriptItems:addAll(getScriptManager():getItemsTag("Fish"))
-- Get Base Vanilla Fish
addExistingItemType(scriptItems, "Bass")
addExistingItemType(scriptItems, "Catfish")
addExistingItemType(scriptItems, "Crappie")
addExistingItemType(scriptItems, "Perch")
-- etc more Base Fish
-- etc more Base Fish
-- Check for specific MOD to add modded Fish that's not yours
if getActivatedMods():contains("INSERT MOD ID") then
addExistingItemType(scriptItems, "INSERT MODDED ITEM module.item")
end
end

And in your fish items that you want to slice add:

Tags = Fish,

Then change the recipe to:

recipe Slice Fillet
{
keep [Recipe.GetItemTypes.SharpKnife]/MeatCleaver,
/*---- This will call your custom function to get all Fish ----*/
[Recipe.GetItemTypes.Fish],