Yes, we can say Swaminarayana is a sub-sect of Sri Sampradaya of Vaishnavism. Swami Ramananda was Guru of Sahajananda Swami. Swami Ramananda told that Ramanujacharya once gave him diksha (initiation) in dream and hence he adopted Vishitadvaita doctrine of Ramanujacharya.
Now talking about scriptures, Shikshapatri and Vachanamruta are two core scriptures centrally in Swaminarayana Sampradaya. Sikshapatri is the only scripture believed to be written by Sahajananda Swami himself and Vachanamruta contains important preaching of Sahajananda Swami and is written by his close chief disciplines.
He accepted following texts:
- Brahmasutras (from Darshana),
- Bhagavata Purana (out of 18 Puranas)
- Bhagavad Gita, Vidura Niti and Vishnu Sahasranama (from Mahabharata)
- Yajnavalkya Smriti (out of 18 Smritis)
- Vaishnava canto from Skanda Purana
Quoting from Sikshapatri:
I hold the following eight scriptures as superior and true authority on our philosophy and religion,(Sat-Shastras).
The Vedas, the Brahmasutra of Vyas, Shree Bhagvata Purana, the Vishnu Sahasranama,Shreemad Bhagavad Gita, the Neeti narrated by Vidura (all the three from Mahabharata) Shree Vasudeva Mahatmiyam from Vaishnava canto of Skanda Purana and Yagnyavalkya Smriti among Dharma Shastras. (verse 93, 94, 95)
Reading Sikshapatri it sounds that he has mixed beliefs and preaching of Vallabhacharya and Ramanujacharya.
Endorsed worship practice of Vallabhacharya:
On the question of deciding the dates (days)of fasting, festivals and on the customs of daily service (in the temple ) to Shree Krishna, my disciples shall follow the decisions already made by Shree Vithalnathji, the son of Vallabhacharya,the Acharya of Vaishnavas. (verse 81, 82)
Promoting Ramanujacharya's commentaries:
It should be kept in mind that the commentaries on Vyasa Sutra (Vedanta Sutra) and Bhagavad Gita both by Shree Ramanujacharya are accepted by me as the sole authority on my philosophical and spiritual decisions. (verse 100)
Doctrine (Vishishtadvaita) said by Ramanujacharya whereas Lord (Krishna) and supreme adobe (Goloka) said by Vallabhacharya:
All my disciples shall understand that my school of Vedanta is Vishishtadvaita of Shree Ramanuja and my beloved adobe is Golokadhama and the Mukti one shall desire is to get the divine body in the Dhama just like that of Brahman and to render service to God Shree Krishna.