Difficult to say for certain. Having 2 different width wheels is not usual and may cause problems if you have a punture.You either carry an emergency tyre repair kit, ie glop and a compressor or 2 spare wheels.
I believe that a Mk 6 Fiesta has an et of 38, ie the wheel sits 38 mm towards the outside of the hub from the centre line.
What you propose is an et of 20 so that means that they sit only 20mm from the centre line., but the width is 50 mm wider at the front and 75 mm at the rear, assuming that the original wheels were originally woud have been 6inch at the most. ie 1.6 sport or ZS.
This means if my calculations are correct is that the wheel will sit 32mm closer in to the fromt suspension and 58 mm closer in to the rear suspension.
Whether you have that much spare room I do not know, you may be better to consider a et of more than the original 38 to allow the wheels to sit a bit further out.
Being a s you are new to motoring and this may well be your first can may I suggest that you learn to live with what you have or at the least put wheels on your cat that are Ford wheels as fitted to other models, may not affect your insurance either.
You would have to inform your insurance company.
check out this link.
This is a technical problem and one that should really be addressed by a wheel specialist who can advise you.
You don't really want to waste your money on wheels and tyres that are not suitable and that no one else will want.