I am sorry to have been so tardy in response.
No, there should be no system compatibility issues between a Fresenius machine and/or its lines and an Optiflux dialyser ... after all, the FX80 and Optiflow series are both Fresenius products.
But ... that is NOT the question. What does matter is the dialyser performance characteristics! The Optiflow f250NR is a very much more 'oomphy' dialyser - in every way - compared to an FX80. The surface area is 2.5 m square c/w 1.8. The filling volume is also greater, as are the clearance, Kuf and KoA data. In short, it will clear much more, and could - without due care - lead to more rapid ultrafiltration and thus instabilities during dialysis if this is not appreciated by the treating unit.
That is not to say there is anything wrong with either an FX80 or an Optiflux 250 ... they are just designed for different circumstances. See http://www.advancedrenaleducation.com/Portals/0/F250NR%20FAQ.pdf