Yes - you can use the SNAP-AIRATE to interface to a passive speed probes (hall effect).
Keep in mind that the both input channels share a common - pins 2 & 6 - so wire accordingly.
Opto should adjust or re-evaluate the specs on the AIRATE. As I've noted, we've read frequency inputs from a speed wheel (more square than sinusoidal input) with a input amplitude below 300mv.
On many applications the use of passive probes is more of a safety measure verses active (powered) probes that can fail with a blown fuse/power supply. You may want to consider using an active probe input which would open more interface options (ie. IDC5-FAST, AIRATE, AIRATE-HFI). We've used all these modules based on our application needs (AIRATE has a resolution of 1hz while the AIRATE-HFI can read a fraction of a hz)