Flipping an 18'er takes skillz. You should be fine. I am basing this on these things:
You have posted video of you running real whitewater in a kayak and not swimming, this means you can read water and have some coordination.
That means you should be able to identify any real obstacles and row well enough to miss those obstacles.
Ruby is class III- at highwater. Set up early, 18' boats are slow, especially loaded down with beer and people.
And while, yes, it is possible to flip a 18' raft in class 2, it takes skillz or more likely a complete and total inattention to the river and probably some level of drunkenness. Float into a hole with 20,000 cfs, you might flip. Run into a rock sideways, you might wrap. Odds are you will just have a good time.