I've seen all most articles in regards to the "Wired" connection possibilities. I'm not concerned about wifi due to unpredictableness of the speed.
So, can the Roku (Ultra)? use a 1gb ethernet adapter?
The Ultra can use Ethernet. It only support 100Mbit, but it'll work fine with a 1G adapter.
But why use an 1gb ethernet adapter if it still only responds to a max 100mb, right?
Exactly. The gigabyte is way overkill for streaming. You'd hardly ever see a stream even at 20 mbps over your local network.