I don’t know why nationalist types are so crazy about Russia, Russia is very diverse. More Muslims than France, lots of different minorities. It even has its own Buddhist republic, Kalmykia.
Serbs and Croats are not ethnically the same. They’re a bit similar because they speak similar Slavic languages. And Slovenian and Macedonian while both Slavic are quite different.
There are also non-Slavic groups in what used to be Yugoslavia, the Kosovars, Hungarians in Serbia, Turks in Macedonia.