I'm not trying to change your plans, but I think that a few people with Cupras that came with 19" alloys have swopped them for 18" alloys to make the ride more acceptable.
I have an Audi S4 that came with 19" alloys which I keep on in summer and use 18" alloys in winter with winter tyres, the facelifted Audi S4 only came with 18" alloys, probably due to complaints from customers.
Don't get me wrong, my S4 feels and drives very well with its 19" alloys on as the S4 suspension is better thought out than the S-Line etc, maybe the same with Leons in as much as what is acceptable with
Cupra suspension might not be acceptable with FR suspension even in its factory state.
Other than that I can't see any outstanding reasons why you can't just slap them on.