Hi Flavio,
1) On mini breakoutboard you can use the otg port (usb). But then you need an external power supply. And you need an adapter to micro usb to connect the soundcard which normally has a standard usb plug.
2) see 1), you don't need the gpio's if using a usb soundcard
3) can't say anything to this soundcard. Maybe not each usb soundcard will work, it depends on the chipset and it must be supported by Linux. I use a Terratec Aureon Dual usb and it works fine.
But remember, the kernel needs the snd-usb-audio module which is not activated in the standard yocto image. Look into the repository of AlexT (mentioned in another thread) and install the module(s) from there.
Regards, mmi