First to set things straight, there are not two Scriptures. One Catholic and the other Protestant. Historically the Old Testament and New Testament canon was decided by a Council of the Catholic Church in the mid 300’s AD. The same Canon of Scripture was re-affirmed by various Councils up to the Council of Trent in the mid 1500’s which was called for two main reasons. To respond to the errors being taught by Martin Luther, and others who were starting new religions, and to reform some of the laxity among Catholics, both clergy and laity. The times prior to the Council of Trent were in many ways similar to our own times, in that many Catholics were not well educated about their Faith. Martin Luther and other Protestants removed books from the Bible that disagreed with their new doctrines, a bit different than the Protestant charge that the Catholic Church added books.
As to Purgatory. All souls in the state of purgation, or final cleansing before entering heaven (being able to see God face to face) will join God in heaven.
The term Holy Ghost is a translation of the German word for Spirit, which sounds like ghost, (German Gheist) because modern english derives some of it’s words from German, the term Holy Ghost and Holy Spirit can be used interchangably and refer to the third person of the Trintity, and not a Ghost, in the sense you are asking.
As to Angels and Demons, yes the Catholic Church holds as a matter of faith that God created Angels, pure spirits without bodies who have certain properties, ie they are not limited by time and space, they were at their creation given full knowledge of God and free will. One of the disadvantages of having full knowledge was that those angels who chose to reject God and His goodness sealed their fate, and when they rejected God, and were exclueded from seeing God face to face, they did so for all eternity, these are Satan (Lucifer) and the demons.
Now as to Ghosts or spirits. God can allow the souls of our deasd relatives and friends to visit us as messengers like the angels, however while this is rarely done. What is more common are several other things. First there are some people who either have a mental disorder, sometimes it is caused by drugs and alcohol, or an imbalance of chemicals in the body, these are Organic mental disorders. There are also others who have psychotic episodes which are caused by extreme stress and other ailments. Lastly there are tricks of the devil and his demons. They can appear to be messages from God, angels, Saints, and our deceased relatives and friends. Because it can sometimes be hard to tell which of these events is occuring the Church disourages such things as trying to devine, or contact the spirits of the dead. If someone thinks that they are being contacted by the dead, they are encouraged to discuss the episodes with their parish priest who will help the person to understand what and where the alleged apparitions are coming from. If the local priest cannot determine there are those who have more advanced training on how to determine this who will be called in to help.