i like to be republic first than switch to communism, i like a lot of cities. i have noticed that other civs are even with me on science and start to get a lil ahead after i have switched to commy.
In an Always War game, Monarchy is the government of choice since it does not have war weariness. Playing AW, you never make peace after you meet someone. It always war all the time.
For almost everything else, Republic is the government. War weariness is a factor, but short, decisive wars can avoid that issue, along with plenty of luxuries and markets in the core, productive cities.
Both of these are available in the Ancient Age.
The issue with other forms of government is that normally you would not stay in despotism until you learned that needed tech. In turn, that means a period of anarchy to go from despotism to monarchy/republic and then a second round of anarchy to switch from monarchy/republic to feudalism/facism/communism/democracy. The second bout of anarchy is sure to be longer than the first since your empire is sure to have grown by then. That means 8 to 9 turns of anarchy for the second switch.
That might answer the government part of your question.
Switching from republic to communism also slows down your research, too, due to the second anarchy.
Communism affects corruption, but not learning. It will make your fringe cities appear more productive but I think it is a sham. I prefer to stay in Republic, have the core cities build the infrastructure and military that is needed and keep the outer, corrupt cities focused on building, wealth, artilllery, workers, settler and hiring geeks.
As a personal rule of thumb, if a city takes more than 10 turns to make a military unit and I am not in the Ancient Age, well, that is too long. Switch the build to something else; irrigate and rail the flat land around it, get to size 6 or 12 as quick as possible and hire geeks to help with research. If the city gets cranky, don't mess with the happiness slider. Hire a geek instead. It will grow slower; can't be helped.
In vanilla, each geek will add 1 beaker per turn towards the tech being researched. In conquests, that number is now 3. What was a marginal use of the geeks in vanilla is now a powerful tool in conquests.