So what's the difference with Radical Socialist, which iirc the British start as, and Syndicalist?
According to the game itself, (where syndicalist takes the place of Leninism and Radical Socialist as LWR), Radical Socialist allows the government to be more "democratic" and "open".
Though it's important to take these at face value, because some ideologies might not completely match what the sliders have for them (Anarcho-Syndicalists). This is due to the limitation in how they can be put in-game as the most one can do is work around a system meant for a different world's politics.
I'd imagine more realistically speaking, Radical Socialists would probably allow for more local control of industries where Syndicalists might favor more direct planning, allowing labor unions to direct policy in place of a bureaucracy managing state industries in another world.
And how do you get the reds to appear as Russia?
As jamhaw mentioned, there's some room here.
Either you create a very right-wing government (be it a Denikin or Wrangel dictatorship, or re-inviting the Czar) and dissent has to be high, or taking the path of the Mensheviks and getting to a certain event around mid-march 1937, which gives you three options
-Keep the Republic
-Create the Soviet Union
-Make a Liberal constitutional monarchy
If you're interested in actually playing the Soviets, it's best to take the more peaceful path, but be sure to cancel all researches once you enter March 1937.