There was no consistent racial theory that the Nazis endorsed as somehow "official". All decisions were made on case-by-case basis depending on current situation and Hitler's personal preferences.
The Hitler's personal preferences were that he hated Jews, Poles and Russians. He was quite neutral regarding some other nations, including Slavic. He regarded Ukrainians better than both Poles and Russians.
He regarded Turkmens (who are Mongoloid, spoke Turkic language and Muslims) better than Lithuanians and Latvians, who are white, Christian and spoke languages which retain the most features of the ancient Indo-Europen.
He preferred Tatars to both Russians and Ukrainians.
This all was completely arbitrary.
It should be noted that along the so-called "Aryanism" ideology there were competing ideologies of Nordicism and Germanism and racial purity theory, as well as others. Of the first three it seems that Hitler liked Germanism the most while Nordicism the least.
All these were cited in different contexts depending on situation. For example, Aryanism was used to exclude Jews, Germanism - to exclude Slavs and Nordicism and racial purity - to attack the French and Italians, as well as Gypsies.
Another theory was that "heroic" nations, i.e. nations that conquered and enslaved their neighbors should be preferred to merchant nations that expanded by trade and the both should be preferred to the nations that were sometime in their history enslaved themselves.
From this point of view Jews were excluded as both merchant and enslaved nation (enslaved as long ago as Biblical times). Russians were seen as being "natural slaves" because they invited the Scandinavian Varangians to rule them, because the majority of pre-revolutionary Russian nobility and the Tsar himself were of German descent (while the serfs were Slavs) and because the Nazis believed that the very word "Slave" was derived from 'Slav" (in German the both words coincide). On the similar grounds Arabs (as "heroic") were preferred to the British (as "merchant") and African blacks were preferred to American blacks (the descendants of the slaves), warlike Caucasus dwellers, Chechens and Cossacks were preferred to "peaceful" Russians etc.