Sounds like a good suggestion 
Maybe it could be simplified to overload the existing --transfers to accept a list of characters?
Something like:
--transfers SSss
would mean 2 threads sorting by Large files first (SS) and 2 threads sorting by small files first (ss).
Some set flags like size, modified, created, name with both an upper and lower case to indicate descending or ascending sorting (SsMmCcNn) could make it pretty easy for the user to specify how many threads of each type they want.