cassette, RD and chain length ??

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I currently have a 13-29 cassette (that I use primarily for long mtn climbs) with what I assume is a long cage Campy RD. I'm getting another wheel set for use with the same bike, but want to go to different cog ratios for more typical training rides. Since there is concern for chain length, etc., what's the smallest large cog that would be compatible with this set up - would either a 12-25 or 13-26 work?

Thanks