In page 28 of the book "Worship of Sri Ramakrisha"( Sri Ramakrishna Math, Madras, 3rd revised ed.1882),the Rishyadi-nyasa of Sri Ramakrishna-Puja-paddhati is described as:
ॐ ऐं सर्वदेवदेवीस्वरूपाय श्रीरामकृष्णाय नमः इत्यस्य मन्त्रस्य ब्रह्म ऋषिः गायत्रीचछन्दः सर्वदेवदेवीस्वरूपः श्रीरामकृष्णः देवता ऋष्यादिन्यासे विनियोगः
My question is, Sri Ramakrishna appeared in 1836 and left His body in 1886. How could Brahmaa be the Rishi of the above mantra or Gayatri be its chhanda? Besides, the Mantras and the Rishi-Chhanda-Devata-samprayoga corresponding to each are always mentioned in the scriptures and can not be constructed or changed, so far as I know. Then how can we explain the basis of such a mantra?