Since Polybian roman units come exclusively from Latium or from Roman or Latin colonies (of which there are none in Sicily) they can't be recruited there.
As to the mercenaries thing, I think its probably just to show the movement of mercenaries to large military centers where they were likely to find work. They're probably just placeholders to be filled by other mercs who make more sense in central Italy fulfilling that role. The descr_mercenaries file is pretty out of date in .81.
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