How do Flight Attendants at other airlines negotiate staffing on new aircraft?

They don’t. No other Flight Attendant contract has a provision. Management at American, United, and Delta, decides what the staffing will be on their aircraft. The FAA minimum staffing on HA’s planned 787-9 is 7 Flight Attendants. American initially staffed its 787 with 7 Flight Attendants. American management unilaterally changed it to 8 on domestic flights and 10 internationally when minimum staffing proved unworkable. Their management could change it again at any time.