I’ve got a cheap eBay wheel arch roller, which I bought a couple of years back, I can confirm it’s not brilliant.
There would be two problems with using it on the 500. First, the double layer arch lips are pretty sturdy and I think it would struggle to bend them. Also remember that arch rollers will soon make your arch 50p shaped if you’ve got any weak spots / rust in the arches. Second, it bolts up to the wheel studs, and unless your 500 is converted to 4x98, it won’t bolt up.
I would recommend doing it with a hammer and dolly, you can feel your way round the arch and keep the shape better, but obviously it is almost always going to result in paint damage when you take a hammer to the bodywork!!