Which types of exercise are good and which should be avoided? is joint soreness normal in the mid back region?

Depends: While some soreness after strenuous physical exercise is expected, you should adapt am exercise routine that minimizes pain, risk of injury and optimizes benefits of regular workouts. Initial consultation with a PE instructor is therefore very helpful to guide you through the process of do's and dont's. You can also google safe exercise routines, depending on what you want to focus on.

Swimming: Exercise in general are good some exercise might be harmful to some joint some could be harmful if not cleared by a physician swimming is always an excellent exercise including all aqua exercise because it is a low impact and does not harm any joint and could be practiced a year long other exercise for specific joint might have to be cleared by a physician including special exercise for back