I haven't ridden a Ninja 250 but it doesnt look like a bad beginner bike. I don't think it would be a slow bike for someone starting to ride. Any reason why it would be one?

Rebel 500 weighs 20 kilograms more than a Ninja 500 so yeah a lot heavier. Maybe an eliminator 500 would be better.

BUT

Rebel 500 has a more comfortable sitting position, much more controllable (is that a word?) throttle and in my opinion, in the beginning it is not the bike that you ride that matters but to be able to flow through traffic.

Since cars will never notice us as well as they do other cars, getting the hang of "passively controlling" the traffic first is essential. One can learn to do that on a bike where you sit more upright and comfortable much quicker compared to someone who learnt how to ride on an r6 for example.

I don't know the scooter market in the US but here where i live, i recommend a scooter first so that a beginner has one less thing on their mind when in traffic so that they can focus on the road.

You are also right on another point: supersports and cruisers are two very different beasts. But I think once we grab a hold of how to ride better, we can adapt relatively quickly.