I have a Yootheme Mustache based Joomla site that also uses K2 as its main CCK. So far all the K2 articles appear just fine w/ the Mustache template w/ no tuning. ... However, the font size used by Mustache is simply too big and not well contrasted for the K2 Tool module (eg. category list, tag list, etc.). There are several discussion threads on Yootheme's web site, OS Training, K2foruikit.com, etc., so the solution has been identified and seems straight forward. ... But since this is not my core competency, I prefer to out source it.
If you have experience working w/ Joomla, K2 and Yootheme template, I would like to invite you to bid for this gig.