1. Romania, Greece and Albania do not share similar linguistic styles with each other or the slavs.
2. The Balkan Union was literally Lenin's idea of like an "Anti-Imperialist Axis of Resistance" which would be allied with the USSR.
3. You could simply do what the USSR did and have a YugoSlav republic (or just split it up into the constituents if they kept fighting) a romanian republic a greek republic and an albanian republic all in federal union (with the potential of autonomous regions for hungarians in the north)