As per Vishnu Purana: Part 1: Chapter 15, such rules are not applicable for Manus, Ptajapatis and other sages who were responsible for the creation in the very beginning.
MAITREYA.--Daksha, as I have formerly heard, was born from the right thumb of Brahmá: tell me, great Muni, how he was regenerate as the son of the Prachetasas. Considerable perplexity also arises in my mind, how he, who, as the son of Márishá, was the grandson of Soma, could be also his father-in-law.
PARÁŚARA.--Birth and death are constant in all creatures: Rishis and sages, possessing divine vision, are not perplexed by this. Daksha and the other eminent Munis are present in every age, and in the interval of destruction cease to be 7: of this the wise man entertains no doubt. Amongst them of old there was neither senior nor junior; rigorous penance and acquired power were the sole causes of any difference of degree amongst these more than human beings.
The commentetor further says
116:7 'They are removed', which the commentator explains by 'are absorbed, as if they were fast asleep;' but in every age or Yuga, according to the text--in every Manwantara, according to the comment--the Rishis reappear, the circumstances of their origin only being varied. Daksha therefore, as remarked in the preceding note, is the son of Brahmá in one period, the son of the Prachetasas in another. So Soma, in the Swáyambhuva Manwantara, was born as the son of Atri; in the Chákshusha, he was produced by churning the ocean. The words of our text occur in the Hari Vanśa, with an unimportant variation: 'Birth and obstruction are constant in all beings, but Rishis and those men who are wise are not perplexed by this;' that is, not, as rendered above, by the alternation of life and death; but, according to the commentator on the Hari Vanśa, by a very different matter, the prohibition of unlawful marriages. Utpatti, 'birth of progeny,' is the result of their will; Nirodha, 'obstruction,' is the law prohibiting the intermarriage of persons connected by the offering of the funeral cake; to which Rishis and sages are not subject, either from their matrimonial unions being merely platonic, or from the bad example set by Brahmá, who, according to the Vedas, approached his own daughter; we have already had occasion to advert to (p. 51. n. 5). The explanation of the text, however, given by the commentator appears forced, and less natural than the interpretation preferred above.