I should probably let MirageRally speak for himself, but I believe he was going to customize his Honda Grom with the same engine? Sounds like he may be skipping that step. If someone is serious about riding a cycle on the highway, I think one would outgrow a Grom quite quickly.
I also have to believe riding a Rebel 500 will be much safer than riding a small Grom with a 471cc engine installed in it. The Rebel 500 is designed (brakes, suspension, and tires) for that size engine. If someone is not blessed with long legs, the Rebel 300 or 500 are good cycles to check out. I think these are perfect starter bikes, but experienced riders would still very much enjoy them, too!
If I were in the market for a new cycle, the Honda Rebel 500 or CB500X would be at the top my list. I really like parallel twins in that (500-650cc) size range. Plus, Honda cycles are super dependable!
The Honda Grom would be a hoot, but I think the Honda Rebel would be an awesome first cycle for anyone! Almost makes me want to buy one & go riding with MirageRally some day!!!!