fastest gate setup to add 7 integer of 32bits
Your 3-stage adder would likely be the best possible. You need to balance the need for an adder with multiple inputs, and mulitple stages of adding. You could probably end up designing a 7-input adder from scratch, but it would probably be terribly slow due to its complexity.

That said, if your goal is to speed up your SHA implementation, you may sometimes need to take a big-picture view on the system.