Is it permissible to consistently recite the remembrance "I seek refuge in the perfect words of Allah from the evil of what He created, originated, and made; and from the evil of what descends from the sky and what ascends into it; and from the evil of the trials of night and day; and from the evil of every nocturnal visitor except a visitor who comes with good, O Most Merciful" daily for protection from devils, or are the morning and evening remembrances fixed, and it is not permissible to add to them?

The hadith you referred to was narrated by Imam Ahmad and others, on the authority of Abdur-Rahman ibn Khanbash At-Tamimi. It states that on a certain night, devils descended upon the Messenger of Allah, peace and blessings be upon him, among them a devil with a flame of fire. Jibreel commanded him to say: "I seek refuge in the perfect words of Allah from the evil of what He created, originated, and made; and from the evil of what descends from the sky; and from the evil of what ascends into it; and from the evil of the trials of night and day; and from the evil of every night-comer, except a night-comer who comes with good, O Most Merciful." Thereupon, their fire was extinguished, and Allah defeated them. Ibn As-Sunni, Ibn Abi Shaybah, and Al-Haythami have all dedicated chapters to this hadith. There is no harm in reciting it morning and evening for protection from the plots of devils.