The barn where I used to board had a few PVC rails. The only time I saw one crack was when a horse stepped on it. I would prefer wooden rails, but you have to do what you have to do... If you're going to be setting up and tearing down each time you ride - yea, go for PVC. Just boot your horse's legs.
Here is a picture of a skinny at a horse trial I did, pretty sure it was about 6' wide. You could definitely go smaller. P1010250.jpg