According to Brad Sams, Microsoft is trying to expand where Game Pass is available and that includes the Nintendo Switch as well as things such as Samsung Smart TVs.
He says that he cannot confirm how soon it will be happening, if there are talks ongoing, etc. he will only report things he has high confidence in.
He also says that people inside Microsoft haven’t been told that it’s happening “yet” but that doesn’t mean talks aren’t happening. It’s something Microsoft wants and Microsoft and Nintendo have a good relationship with crossplay, Nintendo allowing Xbox Live on Switch, Minecraft, etc.
Any device that has WiFi or an internet connection can connect to xCloud. .. xCloud isn’t going to launch in public preview until the second half of 2019. Game Pass games that aren’t available on Switch will use streaming technology via xCloud to work. The earliest time possible for Switch to get Game Pass would be early 2020.