As I'm almost completely EF, I'm not familiar with A, B, or C, but you can switch the cycle at the top of the page.
Mass XI is the general Ordinary for time after Pentecost / Ordinary time; Mass VIII could be because it can be associated with Christmas season. Some designate Mass XV for that role.
The Credo doesn't have a strict association (as neither do most Ordinaries). I tend to think of Credo VI as more penitential in nature... so I would probably use one of the other variants.
We sing Credo I on Feasts, II in Lent and Advent, IV on Sundays after Epiphany / Pentecost and VI when we have a ladies choir to take the place of the cantors alternating with our Tenors and Basses.
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