You should use Kyrie XVII I believe it is -- the one used for Advent and Lent.

Kyrie XI is used for Sundays after Pentecost. I suppose Sundays after Epiphany would be close enough, since they are closely related (you use the 6 Sundays after Epiphany either in January or November)

Kyrie XVII is very simple, which lends itself to singing without an organ. (Which isn't an accident, of course)

The Credo you use doesn't matter at all. There are no different ones for different kinds of feasts, and they are totally independent of the Kyrie.

Just don't use Credo III every single week and you'll be fine.
Credo I, IV and VI are much better :)