you right ,As some vendors, the dynamic SDCCH feature can be restricted only on callsetup events.((to obtain good QoS regarding CSR)).
So all other SDCCH events(LU,SMS)will operated from the fixed SDCCH resourse.
BUT notice that for border cell,than factor for adding SDCCH is not cell location mainly ,but you need to investigate the LU events rate cross the cell.
thus you are generaly right.