Are modern shlokas, scriptures, views, sayings etc. valid when using them as references? Or are only references from the Vedas, Upanishads, Puranas and other ancient texts considered valid?
It should depend on the subject of the question asked. All the questions here can be classified into 2 categories,
- Questions revolving around Dharma, Mokśa or Adṛṣṭa phenomenon (About teachings of Hinduism)
- Questions revolving around history, archaeology, etc., which can't be answered from scriptures & are basically about meta-Hinduism (i.e, about Hinduism, not about teachings of Hinduism)
Now, For the questions falling in the 1st category, reference(s) to either scriptures or works of a valid authority belonging to one of the sect of Sanātana Dharma must be accepted, nothing else. And works of an ācārya or peeṭha-adhipati of any sect/peeṭha makes for a valid authority. About the questions falling in the 2nd category, researches of a scholar(s) can be permissible as a reference.
- References to Osho's works mustn't be allowed as references here because he is like us only with some following. Quoting him is akin to quoting some content of my blog. You can't quote my opinions as answers here, however published somewhere sophistically, similarly you can't quote Osho, etc. here.
- References to the works of Swami Vivekananda would be allowed as references here because he was the peeṭha-adhipati of RK mutt.
Also, what time frame is considered ancient or non-modern? Can a reference given to prove a point from plays, poems, fictional plays etc. valid? E.g., are Kalidasa's plays considered valid and authentic? Books and scriptures of what period i.e., pre-modern, pre-medieval etc. should be referred to for writing answers?
Consideration of time frame would become irrelevant provided whatever that has been said prior to the last quotation come into affect. i.e, if it's coming from Swami Vivekananda then it can be valid whereas if coming from Osho then invalid.