I think what you’ve heard is that mixing different brands of RAM raises the chance that something won’t go quite right and you’ll get freezes and other errors.
But if you’ve bought your new RAM from someplace that promises to pick out the most compatible RAM for your Mac then they’re probably selling something that works well with what Apple uses.
I added 4 GB to my original 512 and I’ve had no problems. I say go for it, but just keep it in mind that if you have lots of unexplained problems you should take the 512 out and see if that fixes it. I doubt you’ll have any problems, though. Just something to keep in mind in case there is a problem later.