Hello, I wanted to know if it was legal/possible to connect an Arduino straight to the RoboRIO USB host ports. The Arduino is for an LED display I set up. 
It is definitely possible. I don’t know of any rules against it.
We used DIO ports to establish communications between an Arduino Pro Mini and the Rio. We also used a DC/DC stepdown to take 12v from the VRM down to the 5v/3a used by the Arduino. Works well.