All of the characters listed in this story are real. Susejo was indeed some creepy person who somehow had been "connected" to the Sarlacc and did indeed love to tease and torture the Sarlacc's victims. Kithaba was indeed a skiff guard who worked for Jabba. He was indeed knocked off the skiff and pulled into the Sarlacc by its tongue. The insults Boba Fett used against Susejo and the way he escaped are from the short story "A Barve Like That: The Tale of Boba Fett" from "Tales from Jabba's palace".
If you are confused as to why the tentacles opened when Kithaba and Boba Fett insulted Susejo, I will explain what happens. Angering Susejo angers the Sarlacc. When Susejo gets angry, and therefore the Sarlacc, the Sarlacc will open the tentacles grabbing the person who angered them and try to slap them. It is confusing, but I hope I made it make at least a little more sense.
http://starwars.wikia.com/wiki/Susejo
http://starwars.wikia.com/wiki/Kithaba