If Harris were to sign I'd hope he'd play at left mid. Hilson hasn't had much luck there and although Dykes has done a decent job when moved over I'd prefer a left-sided player who was more accustomed to the position, and would offer a little more cover for Marshall. I don't want to see Marshall replaced as he's only 19 and has gifted us with very solid performances, appears to be making good progress and frankly, doesn't merit being dropped. If Marshall were to be injured, suspended or fatigued then I wouldn't argue against Harris dropping in to left-back for a few games with Dykes or Hilson shifting back over to the left.
I've noted left midfield as a weakness since our Raydale Cup game vs Gretna and I think it was at its worst against Partick away, as Jacobs was forced to cover and couldn't readjust. If the rumours are true (doubtful), then Harris will be good enough to play that position at this level.