The secret to making cable selection easy is to properly match your components. Get that right, and its far more likely that you will be able to use one brand. If you need to use cables to fix problems that you didn't take care of by selecting the right components, its just endless trial and error.
In your situation, if you put your system together right, and just can't afford to do all of your cables at once, by all means mix them. If you like the sound of the new cables better than the old ones, your system should sound better with each new cable you swap out.
In your situation, if you put your system together right, and just can't afford to do all of your cables at once, by all means mix them. If you like the sound of the new cables better than the old ones, your system should sound better with each new cable you swap out.