First - is the system completely empty or does it have some pressure in it? That is an important question.

If the system is completley emptly, zero pressure, then there is a good chance that moisture has entered the system. With that you need to repair the leak(s) first, then vacuum the system down to remove any moisture.

If the system does have some charge left in it, then you shouldn't have to vacuum it down. However you should really try to repair the leak(s). I have not worked on the Crossfire A/C specifically, so it is possible for some corrections here.

You are probably correct that the low pressure swich is not allowing your compressor to kick in. I would think this systems pressures are monitored by the ECU or BCM or some module.

I assume you are using 134a cans to refill. Start the engine and turn on the A/C. If the system is not totally empty, adding 1 can will usually be enough to cycle the compressor on and off. If one can will not activate the compressor clutch, try a second. If a second can will not turn on the clutch, you may have other troubles. 2 cans should be enough to close the low pressure switch.

As for gettting it to the proper level, I am not familiar with this car yet. Refer to the manual. And as mentioned, be sure to add a can of proper oil sometime during topping off your system.

To get the exact amount of freon in the system, you need to start with an empty system and measure the amount going in by weight.

I hope this helps. James