I"ve not used the QFX but the ACX5k is based on this same chassis and has similar flexibility with ge/xe port options.
You do configure the port namge ge/xe to match the SFP speed inserted. These can be configured prior to having the SFP inserted or after.
Once inserted the hardware should recognize the correct speed and configure match and allow the port to come up. You can confirm the optics installed:
show chassis pic fpc-slot 0 pic-slot 0
Adjust the fpc and pic slot numbers as needed for your interfaces to check. This will show the type/vendor and other details of the installed SFP.