Your (unspecified) servos might be drawing more than 550mA on startup.
Good thing is that servos only draw significant current when moving (or trying to).
A Mega can control AFAIK 48 servos directly (without any shield).
An Uno is limited to 12.
"This Adafruit module seems expensive to only be able to run 2 stepper motors"
I thought you were asking about servos.
Leo..