@ GLK I personally don’t know enyone from Sidirokastro or Kilkis. I’m from South of Serres as Sidirokastro is almost on the Bulgarian border and Kilkis is to the West.
From what I have read all of the Slav speaking population of Kilkis approx 13,000 were expelled to Bulgaria after the second Balkan war 1913 and today is prodominantly Greek.
As for Sidirikastro it was called Demirhisar by the Turks and had a mix of Turks, Greeks and some Slavs. Again after the Bulkan War most of the Turks and Bulgarians were expelled. Today Siirokastro is Greek with a 50/50 split of indigenous proples and refugees from Turkey and Bulgaria.