A guy here (jimindenver) did some work on that, reporting that mono is affected more (amps output drops more) in low light than poly.
This would not matter in a parallel mix where they just add their amps, but it matters in series where the string has the amps of the lowest amp panel. So stay with poly for your plan to go in series.
You might notice that often, 12v monos have higher Vocs than polys, like 22.9 instead of 21.9, which matters for matching but also for adding Vocs wrt Voc limit of the controller
