The USSR was extremely large, and also, it was discontiguous. Kaliningrad was separated from the contiguous parts of the Soviet Union, and to this day, Kaliningrad is a part of Russia.
I noticed that:
These nations (the Serbs, Hungary, Croatians, Bosnians, Czechs, and Slovaks) were never incorporated into the USSR.
However, the Soviets incorporated the following:
So we see people who deviate a lot from the atheistic, Slavic template, but they were fully Soviet people. We also see a lot of Central European people who weren't incorporated into the USSR, even though there was a lot of cultural commonalities.
What were the criterias that the Soviets used in determining which nations to incorporate into the USSR?
First of all, before 1924 USSR's aim was to conquer the whole world. Moreover, the USSR viewed itself as a confederative union (think EU) of independent states with their own languages and laws. The only common thing was the communism. It was at that time when Armenia and other Middle Asian territories were incorporated into USSR. Therefore, the option to exit the union survived in the soviet constitution till Gorbachev times, when it was executed by Lithuania and then everyone else.
According to Lenin, the state should not bear a political function, other than fighting the class enemies of proletariat. Therefore, all peoples should exercise the right to self-determination within the union. It is at that time when USSR was formed with all their territories except the Baltics, Moldova and Karelia.
Under Stalin USSR annexed the Baltics to act as a buffer between Moscow and Germany (it is 600 km between Moscow and Russian-Latvian border). USSR controlling Baltics was a safeguard against Germans because USSR was a whole Baltic Sea away from Germany proper. It was a desperate attempt to make German invasion into Russia harder, as was the whole Molotov-Ribbentrop pact.
After WWII it was specifically outlined that USSR would not annex any lands that are not ethnically composed of peoples already in the soviet union, except for Konigsberg. Lithuania got Vilnius, Ukraine and Belarus got their western territories, Moldova was created out of Transnistria which was in the Russian empire for a long time and had a population of Moldovan speakers and ex-Romanian lands of Moldova. And that's all basically. USSR agreed with western powers not to extend any further.
So we see people who deviate a lot from the atheistic, Slavic template, but they were fully Soviet people. We also see a lot of Central European people who weren't incorporated into the USSR, even though there was a lot of cultural commonalities.
USSR didn't just pop up out of nowhere. It emerged from the ruins of the Russian Empire after a long and bloody civil war. So, a lot of the places you mention were just already there 'by default'. In fact, there are not too many places USSR annexed that weren't parts of Russian Empire before. Notable exceptions would be Galicia (now in Ukraine, formerly Austria-Hungary) and of course East Prussia. Speaking of East Prussia, annexing it didn't make USSR discontiguous, as there was Lithuanian SSR already.
As for other countries:
- Serbia, Croatia, Bosnia, Slovenia, Macedonia, in no particular order were known as Yugoslavia back then, and were lead by a certain Tito, who didn't have much love for the Soviet leadership. So little, he started the Non-Aligned Movement.
- There was certainly no love for the Soviets in Hungary and Czechoslovakia after their rebellions were brutally crushed.
To summarize, your question is anachronistic, because most of its premises are based on the modern state of events.
