This is a really good question, which picks up on a glaring gap between Nazi ideology and Nazi pragmatism. The short answer is that Nazi Germany was absolutely driven by it's racist ideologies, but it also needed allies in order to win a world war. The most prominent examples are probably the Ustase, a Croatian fascist party elevated to power under German occupation of pre-war Yugoslavia, Slovaks (who were also granted a measure of self-government) and Bulgaria, an entirely Slavic country which was officially allied with Nazi Germany. These powers helped the Germans maintain control in Eastern Europe, and shared some (or many) ideological impulses, so were considered acceptable allies, at least for the course of the war.

Looking more closely at Nazi anti-Slavic ideology, however, reveals that there was never a clear and coherent policy towards all Slavs, event at a theoretical level. Historian John Connelly has an article called "Nazis and Slavs: From Racial Theory to Racist Practice" in which he notes that "policies toward Slavs suggest an erratic relation between ideology and practice" [p.2]. Connelly also pointed out that Hitler's writings dedicated little space to dwelling on anti-Slavic feelings, in particular, to anti-Polish sentiment. Czechs drew the brunt of Hitler's anti-Slavic ire, apparently, and this ethnic group made it through the war in relative peace [p.3]. Connelly concludes that opportunity and ideology shaped one another when it came to policies towards Slavs, which were marked by constant improvisation. Nazi anti-Jewish policies, on the other hand, were ahistorical, inflexible and all-consuming. [p.4]

In fact, Nazi thinkers never even came up with a coherent working definition of who was and was not a "Slav". There was also an informal hierarchy among different Slavic nations--Russians were below Ukrainians, for instance--but although it was reflected in some of the treatment different groups recieved during the war this was never officially codified. (see Connelly, p. 10; Karel Berkhoff, Harvest of Despair) When there was an opportunity to play different Slavic groups against one another, Nazi officials were willing to take sides. Connelly offers examples of Ukrainians in Poland receiving support up to 1941, as the Germans wanted to play this minority off against the Poles.

Over time, leading Nazis apparently came to the conclusion over the course of the war that there was no such thing as a single racial category of "Slavs". Conveniently, they suggested that those Slavic groups they had allied with (Bulgarians) likely actually had some "Turkic blood", and that blonde-haired Ukrainians probably had Germanic blood some generations back, which could indicate the potential "germanizability" of some Ukrainian people. Within occupied Poland, some German officials drew on Polish scholarship to tie regional groups to differing racial traits, suggesting Poles could be subdivided into different races. [Connelly, pp.17-19] In short, when convenient, theories about the "Slavic race" broke down. In contrast, antisemitic theories were well-developed and recieved much attention in the press and in books (while remaining entirely conspiratorial, illogical and disconnected from facts about the world, it is essential to remember).

In the case of the USSR, anti-Soviet impulses were inseparable from anti-Communism. Communism was in turn inherently linked to Judaism in the Nazi conspiratorial worldview (an idea usually referred to as Judeobolshevism or Judeocommunism). This was predicated on the idea that all Jews were Communists, and all Communists were influenced by a "Jewish" idea, and so were some how ideologically Jewish. As a predominantly Slavic nation, but also a Communist one, Nazi sentiments against the USSR were driven by an intertwined mixture of hatred for Communists, Jews and Slavs, but it is impossible to pull each of the hatreds apart in a meaningful way. "Russia" (the USSR) also carried ideological connections to Asia in the Nazi mindset, and "asiatic" was a frequent insult attached to description of Soviets. Yet the Germans allied with Japan, showing once again that racism and the practical implications of racial policies could differ wildly.

Furthermore, different high-ranking Nazis had different ideas about how to interact with Slavic people. Alfred Rosenberg, for example, Minister of the Occupied Eastern Territories and the leading Nazi thinker on the issue of Slavs, encouraged an alliance with Ukrainian nationalists against the Soviet government. Hitler disagreed, and so no formal statehood offer was ever extended. But some documents created by the invading German army (Wehrmacht) reveal a much different attitude towards Jews and non-Jewish Ukrainians. In one propaganda report, for instance, drawings were included of happy, healthy, blonde Ukrainian peasants working away in the fields. Though patronizing (and idealized--Ukrainian peasants were not all having a jolly time, to put it mildly), these images differ markedly from caricatures depicting Jews the same part of the army was distributing, which made Jewish people look ugly and frightening. Throughout different levels of the Nazi Party and the military, therefore, different degrees of anti-Slavic sentiment were possible, while anti-Jewish sentiment was fairly uniform.

So tl;dr: the Nazis lacked a coherent racial theory to define Slavic peoples, and improvised during WWII for pragmatic reasons.

The nazi racial policy was far less organized as a political idea than say the concept of "living space". The nazis use race as a reason for blame and action against certain peoples to achieve their own political goals. there was a nazi official Albert Forster in charge of the Danzig west Prussia nazi district in poland that was charged with removing the Slavs; so he just stamped their papers and magically made them German. Nobody said "woah there fella, you have to murder them." There is a lot of evidence of "working towards the fuhrer" where Nazis did what they thought would make hitler happy. (The nazis,Rees)

"The Eagle Unbowed" by Halik kochanski does a great job of showing the chaos that came with the invasion of Poland and the fractioned political environment throughout the area and how some people fought with the Germans , although it's not an easy Sunday read