318/320 depot moves are quite common, though a 314 has dragged a 318 before, see here:
The consist I remember was the 314 at the front, the 320 in the middle, and the 318 at the rear. I think the set came from Sheilds, and arrived at Central Station, as I was at there at the time when I saw it.
The 314 (I think it was either 314 203 or 314 208 from what I can remember, as it was in Saltire Blue) was split off at Central, and would later work a train to Gourock. While the 318 & 320 departed at the front in the 6-car formation to Lanark.