You seem to have the answers you were looking for but, at this late stage, I'll throw in my thoughts too......

With a tartan kilt, I would always, yes always, opt for a plain colour scarf to colour co-ordinate with it - I would never mix my tartans, no matter which they were!



With a plain, non-tartan kilt, I would almost certainly select a tartan scarf, to co-ordinate if possible and to add a colour accent when required (it depends upon the kilt!)