Frankly, your situation could be good or bad for your choices. During this period, the Finns went through four (4) uniform changes: M/19, M/22, M/27, and M/36
Here is a rather helpful page with color pictures of the uniforms themselves:
http://www.mosinnagant.net/mosinnagant.net/finland/tunicsection-photo1.asp.
Note that the M/19 Uniform design follows closely to the German 1915 pattern tunic. The later M/22 pattern appears quite similar to the Russian WWI pattern gymnastroika. The M/27 pattern looks a lot like the M/36 as seen in use during WWII only in a different color of fabric.
Here is a great post-WWI picture of a mixture of uniforms just in one mortar platoon:
A unit of White Guard quite likely taken after 1936 and before 1940:
If I were to tackle the figures, I would go with WWI Germans and WWI suitable Russians shying away from the later M/27 and M/36 uniforms. I would also use WWI suitable Russian heads throughout for their service caps. By using the earlier uniforms as a goal, you can also use them for the earlier Finnish Civil War period and get more bang for your painting and basing buck!
David