I think roboRio has serial ports, and so does Arduino, so you can send data from the roboRio to the Arduino, and the Arduino can translate the data into something that can be run through a logic gate. RoboRio definitely has PWM outputs, and the Arduino can read that using analogRead() or something like that.
NOTE: I’m a CADer, not a coder nor electrical guy. So if I’m wrong… well… uh… ::ouch::
Unless you need a larger number of ports, there isn’t much you can do on the Arduino that you can’t do with just the 'RIO. We controlled some RGB LEDs on our robot last year using a couple of relay ports and spikes, though we could have used 12V relay modules given the small current draw of the LED strips. If you do need an Arduino, I agree that USB is most likely your comms path of choice, because it is supported by hardware buffering and canned software classes on both sides of the connection.
It has both USB-A (2x) and USB-B. I would imagine since most Arduinos use a mini- or micro- USB port (I can never remember which is which) you will have more luck finding a cable for the USB-A ports than the USB-B port. That being said, both have built-in libraries for Serial communication, so interfacing the two shouldn’t be too difficult as long as you make sure they are encrypting and decrypting the Serial the same way. (Disclaimer: I haven’t done this exactly before, but I did something similar using a PC instead of a RoboRIO)
Unless it’s the number of ports you need, or your loading some LED library. I would think you could drive them right from the RoboRio. Most LED strips I’ve seen just take some sort of pwm. I would think you could drive them direct.
I think you should check out the RIOdunio. It is a custom arduino designed to talk to the roborio through the mxp port. You mention controlling LEd’s we have a special breakout for neopixle strips on every RIOduino. They are also available from AndyMark.