It is impossible to talk to a fundamentalist about Scripture, because, well, every word in Scripture has to be literally true, or the whole house of cards comes tumbling down.
How dare the spiritual leader alter a single word.
This is why the Catholic Church's approach to Scripture is so refreshing. I don't have to rely on YOU to interpret Scripture for me or your brother, who may have an entirely different interpretation than you.
The New Testament wasn't canonized until 350 years after John wrote the book of Revelation. So when he states that,
"I warn everyone who hears the words of the prophecy of this book: If anyone adds anything to them, God will add to him the plagues described in this book. And if anyone takes words away from this book of prophecy, God will take away from him his share in the tree of life and in the holy city, which are described in this book."
he is speaking of the book of Revelation the book he was writing.
The New Testament canon that you use is the canon that was determined by several Church Councils in the late 300s. The Old Testament canon that you use was supposedly determined by a group of rabbis meeting in Jamnia around the year 100 the same group of rabbis that rejected the New Testament altogether.