You mention an adafruit shield motor
What is that? Is it a motor or a motor driver shield?
As slipstick mentions, do try controlling the speed of the continuous rotation servos with different servo.write commands. But the electronics in the servo, designed to make positional (non-continuous rotation) servos behave best for positional awareness. When they are repurposed for continuous rotation, They give the ability to move the servo in both directions and stop. But not very accurate speed control.
If the servo.write() works for your needs, then great!
If not, you could consider bypassing the servo electronics and going directly to the motors in the servos using an appropriate motor driver. Maybe even something like and adafruit motor shield... But Pololu has many DC motor drivers also.