will focus rs wheels fit a fiesta st or will i need different tyres

They will fit but you will need the wheels machined to the correct offset.

and different tyres to fit the different sized wheels

my mate adam used to have a set on his ST. and they used too rub a little, as Micky has said your need to get them machined down to stop the rubbing - but if your not worried then they will go on.

I have to admit they did look awesome on his blue ST

Dal

where did they rub

They looked awesome on Adam's old car..

and I bet they were easy as to clean...

yeah they look easy to clean lol and they look really good and solid on the car if you want a set id go for it personally