*Have not read the whole thread*
There are two species of Sulphur-crested Cockatoo. There is Cacatua sulphurea, Lesser Sulphur-crested or just Yellow-crested Cockatoo, and Cacatua galerita, Greater Sulphur-crested or simply Sulphur-crested Cockatoo.
Both have four subspecies each. (And the C. sulphurea have two that may or may not be real subspecies, C. s. djampeana and C. s. occidentalis.)
Cacatua galerita galerita - Lives in mainland Australia, and is the biggest of all Sulphur-cresteds, with a lenght of 50 cm/20 inches, and weighs 700-1000 grams. The crest is often of a more "pointed" shape when lowered, and very fancy when raised. The head is small and the eyes are small with thick, white eyerings. It also often has a very pointy beak, unlike other Sulphur-cresteds.
Cacatua galerita fitzroyi - Lives in mainland Australia, is about 48 cm long and weighs about 600-800 grams. Not much can be found about these.
Cacatua galerita triton - Lives around New Guinea and other islands in Indonesia. Is about 46 cm long and weighs 600-700 grams. The crest is very long and rather "straight", the eyering is a clear blue, and they often have prominent neck feathers, not unlike those on Moluccans and Umbrellas, but not found in any other Sulphur-crested.
Cacatua galerita eleonora - Lives in the Aru islands in Indonesia. 44 cm long, weighs around 500-600 grams. It is rather "neutral" in appearance. No bright blue eyering, no "special" crest, no prominent neck feathers, no cheek patch, etc.
Cacatua sulphurea abbotti - Commonly mistaken for C. g. eleonora, and extremely rare in the wild. Found in Masalembu. About 37 cm long (?) and weighs less than an Eleonora, but more than the smaller C. sulphurea. Both this one and C. g. eleonora are called Medium Sulphur-cresteds, and are almost identical. (Making me wonder why they are placed in different species.)
Cacatua sulphurea citrinocristata - Easily recognized by its unique, orange crest. Found in the Lesser Sunda Islands. Up to 35 cm long, weighs around 360-420 grams.
Cacatua sulphurea parvula - Found in Nusa Penida and Lesser Sunda Islands. Also called Timor Cockatoo, it is similar to the Abbotti, but smaller. Does not have any prominent cheek patches.
Cacatua sulpurea sulphurea - Found in Sulawesi and the Flores sea, this little cockatoo (33 cm, 310-380 grams) is easily recognizeable by its relatively small crest, large beak, and prominent yellow cheek patch, unlike the other Sulphur-cresteds.