Were the Gospels considered Scripture when the rest of the New Testament was written? When the New Testament was written1, were the books of Matthew, Mark, Luke and John already considered Scripture?

1 Acts - Revelation. Obviously there is overlap in when these were written (they are not included in the NT in chronological order), but were the known Gospels considered Scripture?
Given the clarification of the question, an entirely new answer is in order.  

The quick answer is that most of the rest of the NT


was written before the Gospels, and
was sourced by eyewitnesses in any event.  Every writer was in some way written by somebody who had first-hand knowledge of Jesus.  As such, there would have been no need to go back and read somebody else's book.




The dates of the New Testament books can be found in this question.


The entire Pauline corpus (Romans - Philemon), if authored by Paul, predates the Gospels.  

Paul was executed in either 62, 65, or 68 AD, and all of this excludes John (95), Matthew (post-70), and Luke. 

Mark may have been been written prior to Paul's execution, but even given the earliest date for Mark and the latest for Paul's execution would have given only 10 years.  In any event, many eyewitnesses to the Resurrection were still alive at this point, and so Mark would have just been one more eyewitness.  

The closest one could come to making a case for Scriptural recognition of Mark would be Paul's use of the Eucharistic formula, "Take, eat, this is my body," but again there, the question remains whether this was simply eyewitness account or drawing from the book.  Most scholars believe it to be background knowledge of the account, rather than Paul having read Mark's account.  (For that matter, Paul probably learned about it directly from Peter (Galatians 1:18), who is often seen as Mark's source!)
Additionally, James and Hebrews pretty clearly pre-date the Gospels, so again, no.
Acts, which chronicles the life of Paul, was written by Luke, and is clearly the second half of Luke-Acts.  

When Luke begins Acts, he speaks of his own Gospel as "my former book". As such, Luke, when writing Acts, probably was unaware of his own inspired status, but he surely believed what he wrote to be an accurate account of the life of Jesus.
The only candidates for NT books after the "Synoptic Gospels" (Matthew, Mark, Luke) are the Johannine (1, 2, 3d John) books and the Petrine Books (1 &amp; 2 Peter, and Jude which clearly borrows heavily from 2 Peter)

Of these,


John, if it is the same same, did author his own Gospel, and so again, we have the "are you aware of your own Scriptural status" problem.  If it is a different John, the author is still closely indebted to John the Apostle, and so would have considered the John the source, not the Gospel.
Peter, if written by the Apostle Peter (and yes, some doubt whether or not both books were written by Peter), would have had the same status of John.  John was an eyewitness, Peter was an eyewitness.  Peter would have been more influenced by what he saw than what he wrote.  Furthermore, Peter is most commonly ascribed the source of Mark's Gospel, so again, his own self-awareness in regards to inspiration is doubtful.


What may be most useful in answering your question is what Polycarp - a Christian matryr who died in 150AD, thought.  He considered Matthew, Mark, and Luke to be Scripture, along with Acts, the Pauline, and Johnanine corpuses.  Polycarp was a contemporary of John the Apostle, the last of the Apostoles to die.  He's about as early a figure as you are going to get, and he's already really close to the actual writing.  My guess is that he didn't include John in his recommendation list, either because he was so closely associated or it was really too recent. 

Finally, 
Remember that one of the classic marks of NT canonicity was apostolic claim to authorship.  In other words, every one of the books was, at one time or another, considered to have been written by an apostolic eyewitness of Christ.  This means all the works of the NT would have been written


at the same time (more or less)
by people who had some knowledge of each other
first-hand knowledge of the Christ whose life inspired the books in the first place.



tl;dr>
No, the authors of the epistles didn't see the Gospel books as Scripture - they considered the eyewitness accounts as Scripture. Most of the books you are talking about were authored before the Gospels, but while many of the eyewitnesses were still alive.