Um, what "ethnic clashes" among the Orthodox?
The Greeks and Serbs have never fought against each other over anything. They've always been best friends. In 1999, when the Serb army was forced to withdraw, Greeks even came to Kosovo as volunteers to act as bus drivers for Serb children in order to protect them from the Albanians.
Serbia and Montenegro have had political issues -- mostly instigated by the West-- but have never had "ethnic issues" because when it comes down to it, any Montenegrin who was ever worth a damn in Montenegrin history, considered himself "a Serb".
In the Balkan Wars. the Bulgarians fought the Serbs and Greeks over the division of land as the Ottoman Turks retreated, but it wasn't "an ethnic clash" --it was political.
And Macedonia never really fought anyone -- again the the problems between them and Greece & Serbia were political -- also again, mostly instigated by the West.
The Albanians, on the other hand, have no Balkan friends at all -- and for good reason.
I was referring specifically to Montenegrin and Macedonian nationalism (ie, the Macedonian Orthodox Church), and the sometimes heated quarrel between Macedonia and Greece (the Greeks not even wanting them to use the name “Macedonia”).