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According to your info mixed units (Kepi and Adrian helmet) were not possible on the eastern front. This means I have some serious headswapping on my hands (I'm gonna give some of the WWI Hat figs Adrian helmets to create more variety in my shock unit).
What about mixing the summer and winter uniform (units on the Eastern front with some men wearing their summer uniform and others still wearing their winter uniform)???
Did this occur ???????
p.s. I think you could also use the Russians wearing an Adrian helmet for the Saloniki-Thessaloniki front (I think I have seen some pics but I'm not sure).
I think that regulations were one thing, but at least on the Eastern Front there might have been mixing of headdress among units, especially in 1917 as supply became even more erratic. On the other hand, all the photos that I have seen of Russian troops in France and the Salonika front (glad you mentioned it, because the Adrian was also worn by those troops) showed the soldiers with uniform headgear. And, if you want to use these guys in RCW scenarios, feel free to mix heads, as uniformity there was either minimal or else mixed among units in many cases.
As for a mixture of winter and summer uniforms, I'm not sure if I have seen pictures of WWI combat units with a mix but, again, it is probably not impossible (especially if you are depicting two different units that might have gotten mixed up in a fight). Again, in the RCW this was a common occurrence, especially among irregular and guerrilla units.
I wonder if these units presage some Red Army infantry for the RCW in their distinctive 1919 and 1921 uniforms ...