I'm going to buy mini HiFi system soon, but I don't know how to connect them for better audio quality.
I read that connecting via AUX or Bluetooth will not give the best quality and it depends on smartphone's sound chip.
What about chromecast audio, is it good for this situation?
Given that you are going to use a mini system I don't think you have to be concerned with quality above that of the wired aux connection. Even bluetooth will probably be fine.
Chromecast Audio might be better because it doesn't stream from your phone but directly to the Chromecast. Your phone is just a remote control for it I think. No too pricey either.