Does any one has experience , negotiating a salary in academic/govt job , then that was advertised in job description. As most salary range in University are bounded by pay scale/grade systems ,will it be unethical to ask for better salary and grade ?
Your question implies that you are being interviewed on the basis of your established credentials. On that basis, you should have a good idea of how you compare with your peers in your area of specialisation. The pay scales are pretty much set in stone. That is, for any given grade there is a pay range (usually based on years of incumbency). That is not negotiable. Your best chance would be to be assigned to a higher grade and thus be on an elevated pay scale. Generally grades are grouped and it is very unlikely that you would be able to jump groups. The HR approval for the role, most likely, specifies the range of grades for the position. Going outside of that range would, typically, require a lot of additional paperwork. People responsible for hiring have already gone through enough bureaucracy that they wouldn't be happy to be faced with more.
Unless you are already very well paid and happy where you are, you might want to seriously consider (as other HNers have suggested) accepting the best offer and then working hard to prove yourself and thus be recognised for your contributions and being promoted.