I rode with my wife while she was learning highways. We first found an entrance ramp that basically had enough time to get up to speed before we got off at the next exit. We tried that a couple of times until she was ready to skip an exit. And then again until she was ready to just keep going.
Highways can be intimidating. There's no shame in being nervous about them. Lots of 3000lb metal machines hurtling all around you, with their drivers only paying partial attention. Two things though:
1. Highways are actually safer than side roads. It's intersections and roundabouts that are more dangerous than highways. At least we're all pretty much going the same direction.
2. You can keep yourself out of trouble in most cases on the highway. Always be thinking about how you can keep yourself safe. If there's a car entering the highway, make it a point to not be on the side of him, even if he's still two lanes over. He might just decide to cross those two lanes. To that point, never ride in someone's blind spot, or even next to them if possible.
Some would say go slightly faster than the traffic around you. That's decent advice but, in my commute, they average over 70mph. So my job is to find that little safe zone in between all the women putting on makeup, men changing the radio station, and young idiots texting and facetiming (yes, I've seen it more than once, skype/facetime on the freaking highway!) And then sit there. It keeps me out of trouble.
Tho the other half of your question. I weigh 250 lbs. I got a Rebel 300 up to 80mph on a flat and it had a little more left. Was there any oomph to pass? Not unless I planned on taking 20 seconds to get it done. But since I was in that small car-free zone, I didn't need it.
The Rebel is pretty fun at lower speeds (especially on country roads!), but perfectly capable of being on the highway. I just have to do it slightly differently than I do on my Triumph, or on the FJR1300 I used to own.
Ride safe, friend!