1. RTI is applicable to SEBI.
Visit the SEBI website and click on the "RTI" link available there.
Use the search bar located at the top of this page and search for "SEBI", you will get many useful and informative threads.
2. It does not matter whether some company is Public Ltd. or Private Ltd.
What matters is whether that company falls under the definition of "Public Authority" as defined in Sec 2(h) of the RTI Act: Right to Information Act 2005
3. If there is no other pointer to "Public" or "private" the only way is to check with the Registrar of Companies. (This query does not relate to RTI)