Note: Those SparkFun converters you linked are uni-directional, so you may find that of the 4 channels, you only 2. For each stepper you'll need 3 signals (Step, Dir, Enable), and the signals all originate from the electronics (eg: RAMPS-FD) to the steppers (so they all need to be 3.3V in, 5V out).
There are bi-directional ones where you can use each signal in either direction, which will definitely be more useful (as you need less converter boards). That said, the Sparkfun ones are cheap, and while they don't do 5V->3.3V very well, you won't be using that part at all.
With regards to the servos, as long as the servo outputs are driven PWM or duty-cycle, then yes (basically means they're only using a digital signal).
There are bi-directional ones where you can use each signal in either direction, which will definitely be more useful (as you need less converter boards). That said, the Sparkfun ones are cheap, and while they don't do 5V->3.3V very well, you won't be using that part at all.
With regards to the servos, as long as the servo outputs are driven PWM or duty-cycle, then yes (basically means they're only using a digital signal).