Changing a shock absorber and mount shouldn't make anything 'creaky'. Can you tell which side it's coming from?
It is preferable to change shock absorbers and springs in axle pairs, but most garages are happy to do one side. One new spring can make the ride height uneven, and aftermarket types can be stiffer or weaker, or of a simpler design than OEM. A new shock absorber with a very old tired one can affect handling, but you have to use your common sense. If a pair of shockers have been on a car for 10+ years, they're probably both ready anyway. All other suspension components can safely be replaced singly as required.
As for brakes, it's absolutely essential to fit matched pairs of pads and discs - mixing brake rotors and friction materials can get you into all sorts of bother. Many years ago, a motor factor supplied me with a pair of new, but two different manufacturer's discs for a Renault 5, they were the only pair he had on stock. I fitted them with a set of matched pads, but couldn't stop the damn thing from corkscrewing under really heavy braking. The thing was downright dangerous and it took me a while to twig what the problem was, after I had stripped and resealed both calipers and replaced the master cylinder. In desperation, I replaced the odd discs and it fixed the problem, I never made that mistake again.