It seems a bit hit and miss on whether or not 255/35/18's will fit without fender rolling is this because a 255 width tyre from different manufactures is actually slightly different in size?

 

depends on the offset of the wheel

265/35/18s can fit without fender rolling if the wheel offset is proper

 

yes, some have different widths

 

I will be fitting rota g force wheels 9"x18" offset is 30.

 

Also due to this variation in tyre sizes is it not advisable to run a different brand of tyre on the front and rear? Reason i ask is because i was given 2 new Goodyear eagle F1s asymmetrics and i believe these are going to catch on the rear so could i fit the Goodyears on the front and say fit Falkens on the back?

 

yes, their are serious variations in tire width between manufacturers. I used to run 275 Toyo T1Rs (my fenders are rolled/stretched), and they are the same width as my 265 Advan Neovas that I use now. Most manufacturers list tread width on their websites though, so that's a pretty good source of info to go by.