The RS rack is different, I had a problem with mine, where the bottom arms had been replaced with std fiesta ones, and I couldn't shorten track rod ends enough to get tracking right so I presume rack is longer, I put correct bottom arms on and problem went away, then at a later date I needed new rack, got one exchange from motor factor, You could always phone around as someone who rebuilds racks would do yours if they have none to exchange, the riesta rs rack is also got less turns lock to lock so is a quicker rack, I would made sure I had correct one when I did mine.
Hope this helps