Below some snippets from: Bernhard R. Kroener: Menschenbewirtschaftung, Bevölkerungsverteilung und Personelle Rüstung in der 2. Kriegshälfte (1942-44) [personnel management, distribution of population and manpower use in the second half of the war], in: Das Deutsche Reich und der 2. Weltkrieg, Vol 5.2., pp. 777-995]
In 1944 some 1.255.000 non-Germans served in the Wehrmacht: 763.000 from annexed areas (Alsace-Lorraine, Luxembourg), 370.000 Russian and Italian Hiwis (the latter were members of the Italien forces interned by the Wehrmacht in 1943), 122.000 in ethnical-foreign (“fremdvölkischen”) units. All in all some 13 % of the Wehmacht’s Ist-Stärke. [p. 983]
Appr. 1 Mio. Soviet citizens and soldiers served in German formations and attached units during WW2.
They were divided in 4 categories by the Decree 8000 concerning native auxiliary forces in the East by the Chief of Generalstaff of the Heer (Aug. 1942):
- Hiwis in German units, mostly employed in non-combat roles (estimates from Hoffmann, Müller-Hillebrand and Kroener range from 150.000-400.000 in May 1943, with 200.000-250.000 are said to be being likely. Beside TOE, KSTN authoritation, the whole Hiwi issue has a strong aspect of improvisation)
- Security personnel of the auxiliary police forces, which under different designations, were organized by the Army Groups for guarding their rear area (“Rückwärtiges Heeresgebiet”). (60.-70.000 in May 1943).
- Military security units in the rear area (no data)
- Combat forces (Turk bataillons, Foreign Legions, Cossack and Tatar units). (1943: 170-250.000 men)
- Besides, in the Reichskommissariate there were local (?) security forces under the SS (300.000 men)
- Russian POWs also served in Home Flak Bataillons [p. 989]
According to the Verpflegungsstärke der Kriegswehrmacht 1.Sept. 1943, Wehrmacht [p. 974] provided supplies for
Its Allies, including Osttruppen und Legionen
in the field army and Luftwaffe 670.752
at the home front 207.646
POWs and Interned
By field Army and Luftwaffe 316.399
In the Reich, etc. 530.048
End of 1941Germany took first steps to use Russian POWs as a labor pool instead of letting them die by hunger. At the same time the Wehrmacht in the East started to field the first cossack-hundreds (“Kossakenhundertschaften”) from POWs for the mostly unoccupied rear aerea (“Rückwärtiges Heeresgebiet”) behind all 3 Army Groups. At the same time the Heer in the East also started to employ Russian Hiwis in non-combat-roles in an first attempt to raise its combat efficiency “Auskämmungswelle”. 1942 appr. 50.000 Soviet citizens, mostly cossacks, Ukrainians, Belo-Russians, were employed in non-combat roles in the German Ostheer.
According to the General Staffs plans for maximizing the combat power (Aug. 1943) the new type standard infantry division was to have 10.708 Germans plus 2005 Hiwis Sollstärke. But this was a theoretical plan, retro-sanctioning appr. reality. 
End of Dec. 1942 the foreign units from the East (not the Hiwis) were placed under the just established General der Osttruppen under the OKH. In the Caucausus offensive the Wehrmacht succcesuful recruited non-russian anti-soviet Caucasus people: Until 1945 between 170.000 and 250.000 of them fought attached to the Wehrmacht. At first, recruitment of those oriental Russians was the domain of the German Heer, whereas only the SS had the right to recruit in the Baltic and also mostly did so in the Ukraine. In 1944 the SS also fielded some oriental Russian units, which didn’t for last long.
The Hiwi and Osttruppen recruitment begun out of necessity. Starting in 1943 there were attempts to give it some ideogical and political meaning (Manifesto of Prague, “Comitee for the Liberation of the People of Russia”Russian Liberation Army).
The reliability of Osttruppen declined with the German defeats. Thus, end of 1943 the Wehrmacht contemplated to disolve those units. But what happened was, that Osttruppen security units (Ostbataillone) with then appr. 30% Russian volunteers were switched with those third rate German units guarding the “Atlantikwall”, using a 2:1 ratio.
Apparently the Hiwis embedded in German divisions’ rear service seem to have been more reliable, even in late war. [pp. 986-988]