Assuming the cross-stick shows as its own midi note value, open the Velocity lane. Click on the tiny keyboard(left side of the midi/instrument track) on the note that triggers the cross-stick. Doing that will highlight the velocity value(the tiny diamonds) for all the cross-stick hits. Drag any one of the little diamonds down and all will follow.
Alternately, open the Slate plugin mix window and adjust the cross-stick volume
