For weather it is important to know that oxygen atmosphere makes a difference in terms of health. If people can happily toil outside, they are far more vulnerable to weather. Violent weather is very bad, catastrophic on an oxygen world. Harsh weather is also bad. At unstable it becomes reasonable, but the unstable/oxygen combination is still problematic if you just have starter hospitals.

If you can change weather or atmosphere depends on what you're changing it from. The better it is already, the lower the chances. I'm not sure if there's an actual cutoff for a single terraformer of our current tech, but going from mild to stable or carbon dioxide to oxygen needs a lot of luck, time, or loads of terraformers.

steady > mild > unstable > harsh > violent