Hey, since I got both R.T.F.M. and Hevun's Rebel, which would be the best to start with?

There’s no particular reading order with those stories. RTFM is the origin/explanation of Rael - who is not in Hevun’s Rebel. OTOH reading RTFM first can lead you to connecting some dots sooner. (I won’t spoil them for anyone)

It’s up to you, in the end. All the two stories have in common is that they happen in the same universe.