I still have not seen this movie but did read the book a few years ago.
The situation in Estonia was different from that of Latvia at the time. Most Estonians wanted independence from Russia and there were not many who backed the Red Army. In Latvia and Russia there was a civil war, Whites vs. Reds and many different factions representing the Whites or anti-communists. Most of the Russian soldiers had been conscripted into the Red Army and many had changed sides 3 or 4 times during the war. The Russian soldiers who were captured by the Estonians were given the opportunity to join Yudenich's White army and many did. Many Latvians were ideological communists who were fighting for an idea and there were also quite a few Latvians who fought under the Germans in the Landeswehr and later in Bermont-Avalov's army. At one time Latvia had 2 White Latvian factions, 1 Red and 1 pro-German faction. The Latvians only defeated the communists because of the help given by the Estonian government, a fact that Latvians do not want to talk about these days.
If you ever get a chance to go to Latvia, check out the war museum in Riga. I learned so much about the Latvian Civil War there and was amazed by how divided the country was at that time compared to Estonia.