1) The Linksys USB100TX are the currently recommended adapters for both the Rio Receiver and the Rio Central, yes?

The Rio Receiver doesn't need a USB-to-Ethernet adaptor: it already has Ethernet, and doesn't have USB! And if the only reason you want networking on your Rio Central is to attach Rio Receivers to it, you can do that with HomePNA (telephone cables) with no extra adaptors at all.

2) Recommendations for speakers for the Rio Receiver? It says it's got a 10W/channel amp. Not sure what's appropriate for that.

10W/channel is more than it sounds. Any bookshelf speakers should be fine. We've used them with quite big Mission floorstanders and they were fine too.

Peter