When should a question be considered offtopic related to altcoins? And are there any rules for answers that mention specific altcoins when it can be avoided or not mentioned in the question or considered controversial/unethical/scam?
The word "altcoin" is not outright banned (nor should it be) and questions and answers may mention altcoins if the question/answer is generic enough to be applicable to Bitcoin. Questions and answers are not removed for simply mentioning or discussing altcoins. They are only removed if they are only applicable to altcoins and not related to Bitcoin at all.
There are a few meta questions about this site's scope. The most relevant is probably the proposal to change the scope to just Bitcoin. You may be able to find better and more complete guidance through there and the linked discussions.
Are the rules same for everyone including moderators?
Yes. Are there any instances where you think that a moderator's question/answer was not deleted but a similar one from a normal user was?
If we are allowed to mention specific "altcoins" on "bitcoin" Stackexchange how do we decide which altcoins are good enough to be mentioned?
IMO any altcoins can be mentioned so long as it is relevant to the question and the question is also relevant to Bitcoin in some way.
For example, if there were a question about why Bitcoin doesn't implement a feature in that is present in altcoin X, I think it is perfectly reasonable to have such a question on this site and for altcoin X to be named in the question and answer. Such a question is relevant to Bitcoin and the altcoin is relevant to the question and answer.