The region of Macedonia in Greece didn't have a Slavo-Macedonian minority for quite a while... In 1926 for example, the place was 88% Greek, and it only increased since.
http://en.wikipedia.org/wiki/Macedonia_(Greece)#Demographics
Moreover, the particular town Kastoria doesn't have any Macedonian minority that I know of - of course I could be wrong, but geographical proximity to a country/language doesn't guarantee the existence of a minority, and I think this is the case here. :)
Anyway, after, and during the Balkan and World Wars, many Balkanic countries engaged in population exchanges in order to "purify" their territory, and Greece was definitely not an exception. One thing is for certain - nothing is certain in the Balkans! ;)