You're overlooking something very obvious in your criticism of Paul's letters:
"Paul said about fornicators, idolaters, etc. that they should be booted out of the congregation and should be "treated as a person of the nation, or a tax collector" and that no one should "even say a greeting to him." "
Obivously Paul was referring to those that continued their sinning ways although they were brothers, in other words, they were not repentant of their wrongdoing.
"But who did Jesus eat with? Prostitutes, people of the nations and tax collectors"
Obviously Jesus could analyze their hearts. You really think that he would accept a Prostitute if after he tought her, she would go out of the home and continue prostituting herself. Jesus accepted only those that were repentant and changed their ways. For example, look at the apostle Matthew. Also former prostitutes changed their ways after accepting Jesus' teaching since the Bible also mentions that some of them would follow Jesus as he tought multitudes.
In conclusion, Paul's letters ARE in harmony with the teaching Jesus Christ.