55? a pair of each i agree would need length for territories, but neither get very big. im suggesting a 36 long preferably, but even then i think its spacey and id get bored with such small stocking. but u have to work with aggression, so i dont think cockatoos and rams are very good together.
if u had only rams then u could have a smaller tank, cockatoo colonies need bigger tank space as they are way more aggressive.
whats the ideal tank u can get, why these fish? x