I have been wondering whether ancient historians, like Thucydides or Plutarch could be considered as reliable secondary sources or should be regarded as primary sources. As a rule of thumb,a secondary source is a work that interprets a historical event or period after the event has occurred and, generally speaking, with the use of primary sources. So, Thucydides and Plutarch, should be consider secondary sources But considering them as such, seems awkward. What is your opinion on that?
Generally speaking, medievalists and classicists consider anything written broadly speaking in our periods to be a primary source. It's actually a useful lesson for all primary sources, in fact. No source is the thing its text or image depicts; all sources must be interpreted through layers of context, motive, means of survival, and so forth. We learn ways to interpret sources that describe events decades or centuries previous; yes, different scholars can have different ideas about how much we should trust a given ancient/medieval author.
Usually, the term "secondary source" would imply a stance of relatively modern (19th century+) academic perspective. Or rather, the ideas that "secondary source" connotes would imply that: a modern scholar piecing together older sources to analyze and understand the past.