I'm sure you meant to write that Poles wore military clothing of whatever nation under which they were living at the time. Poles were never merely Russians - bite your tongue! They remained Polish despite living under German/Prussian, Austro-Hungarian or Russian governments.
On the whole true, but not entirely. Some people at the time born in Poland who spoke Polish disagreed with you. Denikin was more Polish than Russian, but loathed Polish separatism. Likewise after 1917 Dzerzhinski acted entirely like a Russian who happened to speak Polish.
From a Tsarist point of view there were members of the Empire who spoke Polish at home, but there was officially no such thing as a "Pole", and they went out of their way to prevent Polish being regarded as suitable for any public activity.
I would be interested to know how the independant polish army after WWI absorbed their national contingents before they kicked the ass of the bolsheviks.
They didn't, on the whole. One of the great failures of the 1920-1939 Polish state was its inability to deal fairly with its minorities.
The Poles were not opposed to ethnic units. They fielded a unit of Polish Muslim Tartar cavalry. Bulak-Balachowicz's unit was mostly Belorussian. There were a couple of Russian Cossack units.
However they had so enraged their Lithuanian, German and Ukrainian countrymen that they got no support at all from them in 1919-1920. (The units in the Polish army noted as Lithuanian-Belorussian were actually ethnic Poles from those areas.)
And the less said about their treatment of the Jews, the better.