The setting of the parameters of the expander is similar to setting a gate depend on similar factors. The biggest one, is the range of sound levels that the snare is going to be played at, during a performance or song. If the level is pretty uniform, then I would set the input gain control and tone controls to give me the sound and level in the channel I require to minimise, background hiss, without clipping .Moving on to the expander; first set the source to “self post eq”. Then I suggest that you set the threshold just above the background noise being picked up by the snare mike , then set the ratio fairly high to at remove the spurious background noise. Again like with a gate. I like to set the attack near 0 mS so I do not lose the edge and bite. Then with regards to the release , set the it just long enough to let the whole of the body of the snare and the snares through so they don’t sound unnaturally curtailed.
higher knee values help reduce how obvious the expanders action is, if you want to exaggerate the edge and bite of the snare go for a low value .
There is no high pass filter in the expander so you must use the high pass in the snare channel eq to help reduce triggering by other bass instruments ,don’t do this at the expense of keeping the snare sound solid.
If you have a piece of music that requires the snare to be played from very quiet to a substantial volume you may have to set the threshold much lower and possibly reduce the ratio , in this case you have to look at the whole and not think of each mike as only picking up one instrument, you must integrate the signals from each mike so they blend together . Its like using a stereo pair ,you expect more than one instrument to be picked up by these mikes but it does not stop you producing a good mix by blending in appropriate amounts of signal from the spot mikes
the best way to apply a gate varies and you must experiment !