OK, I'm not ordained, but I have worked for a PCUSA organization for a quarter century, and attended my share of ordination ceremonies.

A kilt is totally acceptable. Period.

I have no idea about the collar protocol...can you actually wear one before you are ordained is a question to ask an actual clergyman in the PCUSA. I don't recall ever seeing anyone in one at their ordination service, but that may be my faulty memory. Around here the clerical collar is a very rare bird indeed.

I do worry about the "just a shirt" bit. You need to round up a jacket of some sort. The service will not be full-formal, but you also don't want to be the most underdressed in the house, and despite the kilt you'll look underdressed without a jacket.

And for goodness sake, wear the green socks!