

Collatz_problem is correct on both counts for the armed forces and religious IGs. I just want to add that you can use this general cheese on any ideology capable of taking up the communist ideology (so TUs, AF, I think rurals?), so an easy way to get a communist leader of the TUs is actually to have a communist general that is part of the TUs. Once the leader dies/retires the existing characters have a chance to get that role based on popularity and influence. You can actually see those chances if you click an IG and scroll down to the Leader Ideologies drop down.
Yup, Alaskaball already described this process in detail, but the gist of it is you will have an almost all reactionary government at the beginning of the game in almost all countries. As you develop further, the landowners will lose influence as their base of power becomes diluted by industrialists owning more of the means of production. It’s worth checking in on your government every few years to see how the IG power percentages have changed. As you play the game more you’ll get a feeling for how close you are to being ready for enacting the council republic law. Definitely just focus on building up your industry like I mentioned in my first post.
Edit: the religious IG can be made pretty irrelevant if you change the PMs for the cities/universities/government admin buildings.