Hello Joe,
Are you using the digital demodulation measurements in the Spike software or zero span? I am attaching a quick image I took of the software configured for the settings you listed. This might help you configure the software for 2fsk demodulation.
The device decimation is selected based off of the chosen symbol rate. We do allow the user to customize the IF bandwidth a bit, but not enough to change the decimation of the receiver.
Let me know if you have additional questions.
Regards,
A.J.