Lcy-Nyc could not be fully crewed by Citiflyer crew as the pilots would not be able to operate the 318 under the Scope agreement. Having a 'half half' arrangement of mainline flight crew and citi flyer cabin crew would cause a huge bun fight.
The 318 is an aircraft designed (note 'designed') to carry in excess of 100 pax therefore it will be crewed by mainline flight crew and thus mainline cabin crew.
Sorry to pop your bubble.