You can certainly run those motors direct drive on the ballscrews, but at 36V I doubt that you will get 200 IPM reliably if you are using 1605 ballscrews. The linear rails will help having very low drag, but with 1605 ballscrews the RPM needed for 200 IPM is 1020 rpm. That is at the upper limit of what just about any open loop stepper can handle from my experience. Add to this that a 640 oz-in 4.2A motor is going to be around 7mH inductance and would ideally need about 80V. At 36V your torque will fall off faster. I would definitely boost the voltage to the max your drivers can handle (48V). If you were to use 1610 ballscrews then you would only need 1/2 the rpm and things get more tolerable.