Not important to go all one brand. Do try to get similar styles though--don't mix ultra modern looking handles with clunky pro style, for example.

I have never had matching appliances. It's more important to buy the best appliance for the job, imo. You should also place more importance ($) on what you feel is important. For example, you may want to give up on a counter depth fridge for a better range.

Most all stainless or stainless/black combinations look fine together.

Your kitchen will look more professional without matching appliances. Buy the best appliance for whatever function you're needing it for.

Yeah, none of my appliances are the same brand. They're all white (and, while the white of the appliances is whiter than my cabinets, the white appliances all look pretty much the same tone of bright white.)

Function over brand.