Genuine Gurus are rare these days, so this question shouldn’t arise, but for what its worth, these are some insights by Sri Aurobindo and the Mother on the question of multiple Gurus. The gist of the matter seems to be that as long as you don’t feel strongly drawn to any particular sage, it is permissible to draw inspiration (through reading and interaction) from multiple sages. But once you get initiated by a particular Guru, you must stick to that Guru, otherwise the spiritual energies of different Gurus can interfere to create a frightful mess within your consciousness. Furthermore, if you have reached the apex with one particular Guru, you can certainly look for another. The cases of Kapali Sastry and M.P.Pandit can be cited in support of this clause; they were initially disciples of Ramana Maharshi but later choose Sri Aurobindo and the Mother as their Guru.
Sri Aurobindo and his disciples uncovered connections between the Vedas and the later scriptures such as Upanishads, Puranas and the Tantra by tracing the evolution of concepts, use of common verses and the underlying symbolism between these scriptures. This is a synopsis of their discoveries collated from a variety of sources.
This is a simplified explanation of the Vedic theory of Sound (Vak, whose root is Vach which means “to speak” and corresponds in Latin to the word is Vox) using some examples as well as the words of the Mother of the Aurobindo Ashram.
The Upanishads, besides delineating various spiritual experiences, also give a few hints on sadhana, i.e., paths of spiritual realization. These methods of sadhana are called vidyas. This post outlines this in brief.