Hello ecorrales, and thanks for you post!
I assume the Redboard can be powered either through USB , or the barrel connector. What if both are used? Is that an issue?
While you should be OK powering from both interfaces, we recommend sticking to just one to be safe.
I assume the expected voltage via USB is 5VDC ? But more flexible via barrel connector?
Yes, USB is always 5 volts. The barrel jack can accept 7 to 15 volts.
I think ideally I would want to power via barrel connector, and just USB for communications from Raspberry to Redboard
Since the board uses so little current, the preferred method of powering the RedBoard Artemis in your situation would be through your USB cable since you're using that for data anyway. (The Pi really won't notice the few mA's the board needs and it simplifies your wiring.)