The similarities and differences between the Rishis and the Siddhars are very striking and also very difficult to notice, though this sounds paradoxical.
Both are Realized souls.
Both have recorded their experiences with Godhood in the form of Mantras by the Rishis and by Poems by the Siddhars.
Both have established Guru Sishya Lineage.
Both have their Idols installed in temples, though they are limited in Number.
Both were devoted to the welfare of the Mankind.
Both transcend Time and Space.
While only seven Rishis are reported to be be beyond Space , Time and live forever, all the Siddhars are Timeless.
All Siddhars were/are followers of Sanatna Dharma Vedas and a unique system developed by them, The Siddha system, while the Rishis followed on only Sanatna Dharma.
Vasishta, Viswamitra, Agstya are some exceptions.
Siddhars seem to have worshiped only shiva while the Rishis all the Gods.
Though some siddhars like Bhogar worshiped Goddess Bhuvaneswari, Agastya Ganesha, Subrahmanya.
The Siddhars worshiped reality, Nirguna Brahman as sound.light and as Saguna Brahman Lord Shiva.
Most of the Siddhars are from the South of Vindhyas though there are references that the North Indian Guru Parampara came from the Siddhas.
Please read my articles on this.
The Siddhas expressed themselves in Tamil while the Rishis in Sanskrit.
Some Rishis have been advisers to Kings, like Vasishta, no Siddha is associated as an adviser to King.
All Siddhas have been Rishis while not all Rishis were Siddhas.
The Yoga system by the Siddhas is slightly different from the one By Patanjali in Sanskrit, though Patanjali is from South and a Siddha.
Siddhas have developed alternate system of medicine based on hers found in the South, the Rishis , based on the herbs found in the North.