Fiscal number and non-habitual-Residence (Lisbon)

Good morning,
I`ve been asked by Orden dod Medicos to get a portuguese fiscal number, but I'm postponing it, as I still have my job in Spain and don't want to register me in Portugal until I am able to ask for the spacial taxes condition (non-habitul-residence).
Does anybody know if getting a "contribuiente" (fiscal number) can somehow spoil later my chances for the reduced taxes conditions (Residente nao habitual).
Thanks a lot for any advice,

Susana