The Vedas declare the nature of the Supreme Truth in many ways. Sometimes they call It Brahman (the great and glorious essence);
other times they call it Sat (Being); still other times they call it Purusha, other times Vishnu, Rudra, Atma, Paramaatma, etc. How are we to reconcile all these various names? Surely they all refer to one Supreme, as the Vedas declare that there is only one Supreme Cause -- ekam eva advitIyam.
It is beyond doubt that Lord Maha Sastha is the Supreme Being described in the Vedas.
Fortunately, the Vedas themselves offer a reconciliation of all these names, contained in the various Upanishads and even in the text of the Veda proper. In the Purusha Sukta, found in all four Vedas, the Supreme Brahman is described as the being who exists everywhere. This most important Vedic hymn praises the transcendental "cosmic man" – the Parama Purusha, the personification of the all-pervading Brahman- who is none other than “Sastha”
There are even more direct Veda vaakyas on our Lord Sastha.
In the Yajur Veda, we see for example,
“Sastha adhipathir vo asthu” - (Sastha is the ultimate ruler), a reference to the supremacy of Sastha over every being.
The Maithrayini Upanishad Says
“Brahma, Vishnu, Shiva, Sastha, Pranava and Brahman – is one and the Same”.
The Kalagni Rudropanishad calls Shiva as “Sarva Sastha”
These Veda vaakyas clearly spell out the principle that the concept of Sastha encompasses all other deities. Even otherwise, the meaning of the word “Sastha”has perhaps the deepest philosophical significance of any name of God, over and above even the terms Vishnu, Siva, Brahma, Indra, etc. The latter terms respectively mean “pervader”, ``pure'', ``great'', and ``king'', and are applicable to any number of things, including the individual self. However, Sastha means ``One who rules all the creatures'', which by implication can only refer to the Supreme.
Though the Sastha temple at the holy Sabari hills is very famous, it is to be noted that there are innumerable Sastha shrines throughout the country. Sastha is not only the Lord who is in Sabarimalai… Infact Lord Sastha is the actual source for Ayyappa incarnation..
Lord Sastha is the Primary force; He has two consorts – Poorna and Pushkala. Sastha has taken eight divine incarnations for His divine missions. Out of his eight incarnations, the most popular avathara or incarnation is stationed at Sabari hills as Ayyappa (as brahmachari or bachelor).
But Maha Sastha as a principal force, resides in his abode as Poorna Pushkala Sahitha Hariharaputra. Even during the Vedhic age when deities were worshipped only through fire, Sastha had a significant position and was worshipped in stone idols. The Vedhic seers (rishis) who lived in dense forest had Sastha as their ishta devata and that is reason we find numerous Sastha shrines in the forest area today.
Thus, Sastha’s worship who is the Supreme Lord of the Vedhas will bestow health, wealth and all the good things in life one can desire... and also grant moksha after the life.
It is to be noted that that Lord Hariharaputra is the most celebrated Lord among the Vedhic community of Brahmins. He is the Ishta devata, Para devata and Kula devata of them. During the magnificent Sastha Preethi celebrations held at every nook and corner of the world today, one can witness the significant role of the Vedhas in the rituals.
Sastha is called “Vipra pooja” – one who is worshipped by the Vedhic Scholars.
All our saints and ancestors have stated unambiguously that in Kaliyuga, Sastha’s worship is the only way of salvation.
“Kalau Sasthru Vinayakau”- Sastha is the only master in Kali
It is true that the Vedas themselves often praise other gods. However, it should be understood that that the personality to which the vaakyas refer is only Maha Sastha – the ultimate divine force.
Among the most important duties of Vedhic Bramins, the Vedhic Yagnas stands first. Vedic yajnas such as Vajapeya, Pashubhanda, Jyotistoma, Ashwamedha are well known. Out of the forty samskaras a human has to perform, these Vedhic yagnas are to be done at least once in a lifetime. This is of top-most value in the life of every householder. Whatever he gets from his farms, from his trade and commerce, whatever he earns from his job, must be shared with other members of universe.
If one observes the Hariharaputra Sahasranama carefully, it will be evident that Sastha is explicitly described as the Lord who gives the fruit of these Vedhic yagnas.
Namas which clearly explains the Vedhic yagnas which are not to be found even with the famous Vishnu, Shiva or Lalitha Sahasranamas are very clear and continuous in Sastha’s Sahasranama.
From the namas like Dwadasaahakrathu preethaa, Aptoryaamakratu mayaa, Poundareeka Phalaprathaa, Pasubhanda phaladhaathre, Vaajapeyathma Daivatham etc – the only point being made is that the ultimate Truth explained in the Vedhas is none other than Lord Maha Sastha.
Taittria Aranyaka Says:
Anthah Pravishtas Sastha Jananaam Sarvaathma |
Sarvaa: Praja Yathraikam Bavanthi |
"Where all are united, He who has entered within, that are born, and is the self of all, is beyond comprehension, He is Sastha the ruler of all things”
Connection of Sastha with Srividya Upasana 
with utmost secrecy that too only with a few specific schools of Shri Vidya...
You wont find any puranic lore for the relation between these Vidyas and Avatharas but the logic of putting the Dasamahavidya with the Dasavataram is related thru perception and thru the Tanric interpretation.
She is Vishnu and He is Devi; There is no difference between them.