standard practice is that you only have identical heating elements hooked to the same control device, ie all the same racks or all the same pads etc...
at your own discretion you could try and put a dimmer or rheostat on the hotter device to try and bring it down to the lower device. it's going to be hard to keep consistent temps though.
So as long as I get the grounded helix, I can just plug all the heating devices into it as long as theyre the same and they'll all get the same wattage?
if they are all the same heating element, same wattage, and in the same conditions then yes. for example, 3 (or more) racks stacked or side by side can all be controlled by the same device.
you wouldn't want to control one sweaterbox rack and one 3' rack with the same thermostat since the wattages are so different. and you wouldn't want to control two identical racks in different locations with the same thermostat since the ambient temps will be so different.