The FRS offset is ET42 & as far as i know most cossie wheels are around ET15 to 20 - either way with the cossie offset the wheels will stick out of the arches too far & also rub

Duncan (The FRS Registrar on the RSOC) bolted a set of his cossie rims onto his FRS one weekend to see what the fit was like & they stuck out of the arch by a good 1 & 1/2 cm