The greatest authorities of Dharmasaastraas are eighteen in number. Manu, Brihaspati, Daksha, Yama, Yajnavalkya, Gautama,Prachetas, Paraasara, Angiras, Samvarthaka, Shataatapa, Likhita, Shukra, Atri, Vishnu, Shankara, Aapastambha, and Haritha.
Bohaayana, Aapastamba, SatyaashaaDa, Traahyaayana, Agastya, Shaakalya, Ashwalaayana, Saambhaveeya, Katyaayaana are the nine poorva suukthaas. Vaikhanasam, Saunakeeyam, Bharadwajam, Agrniveshyam, Jaimineeyam, Mathrunyam, Maadhyandhinam, Kaundinyam, and Kausheetakam are the apara sootraaas.
The Kausheetaka and Aaswalayana Sutras are attached to Rigvedam.. Bhodaayanam, Aapastambham, Bhaaradwaajam, SatyaashaaDam, Vaikhaanasam... these five sutras are attached to Krishna Yajurvedam. Shukla yajurveda has a sootra called Katyaayanam, and Samaveda encompasses Traahyaayanam and Jaiminiiyam
The fundamental belief if Dharmasaastras is that the Rishis who have discovered the laws have ordained the proper conduct. What they call Dharma is so and if they have probhibited anything categorically, it is adharma.
वेदोऽखिल धर्ममूलं स्मृतिः शीले च तद्विदां।
आचारश्चैव साधूनां आत्मनः तुष्टिरेवच॥
vedo'khila dharmamūlaṁ smṛtiḥ śīlee ca tadvidāṁ|
ācāraścaiva sādhūnāṁ ātmanaḥ tuṣṭirevaca||
The root source of Dharma is the the four Vedas. The Dharmasastras, and the local habits or precedents also are valid insofar as they are not in conflict with the dictats of Vedas. The ceremonies and observations followed by the noble are also part of dharma and the litmus test for dharma is that it should give delight to the self.
Ultimately, the aacharyaas have always permitted amendments to the rules after very strict and impartial evaluation of the pros and cons of a particular rule.