Are the versa rails interchangeable between PE Models?

I have a PE 1800 and a 3rd party rack with round holes.  I have concluded that I need Versa Rails.  I see a ton of these available on Ebay for various Dell server models.  I am wondering if the rack rails are generally interchangeable amongst models, or at least amongst some models?  The PE 1800 is a 5U rack model.  Can I use rails from the PE2900, PE 6850, or other models?  Any help appreciated.