2. When it comes to description versus dialogue, if it helps, think of it like this. Focus more on the action, not the characters' interaction. Characters' actions and concerns become more natural when you make them react to the world around them, rather than interact with each other. So for example, when you have a group of protestors in front of a fire hall, ask yourself what the main action is. Chances are good that the main action is the protest. So it's the protestors' actions that should be up front and center stage, not the actions of the working srains. By focusing more on the action instead of the characters, you encourage yourself to break out of your characters' rigid social circles and examine the world around them in better detail.
Also, remember that your readers are not you, so you'll need to give them a lot of detail so that they understand how your world works, which in turn will give them a basic understanding of why characters act the way they do. Like I said, a lot of the prejudice and hardships srains face would make more sense if we knew the history behind the way they're treated. In this step, it's definitely a good idea to do a bit of research. For example, there seem to be parallels between the srain and the way African-Americans are treated. Why not do some research into the history of segregation in order to understand the kinds of things Nolafus and the srains like him face? Once you figure out how their real-world counterparts function, you can translate that into the way your species functions and begin work on figuring out when it's relevant to bring up. (Hint: For major details, you'd want to bring them up early. Minor ones can be sprinkled throughout the rest of your work, whenever they become immediately relevant.) Once you get started with that step, always ask yourself questions about what you're writing (Example: "Why did they do this?") in order to ensure that you've covered all potential plot holes.