In the 'Red John' Episode 8 the only two guest characters are Reede Smith and Gale
Bertram. So either they decided not to include the real Red John in the guest characters or it has to be one of them.
http://thementalist.wikia.com/wiki/Red_John_(episode)
Okay, so the season finale was designed to throw us all off a bit, however, I think it actually helps make some possibilities more clear. The KEY if you think about it is that RJ seemingly read PJ's mind, and picked up on a happy memory that PJ had never told anyone. Now, unless the show takes some weird supernatural turn, we know that there has to be a reasonable explanation. So what could it be? My guess is that after PJ's wife was killed, he spent some time in a mental hospital before joining the CBI. We saw his therapist, and we saw that he was kept at least for a time in a small room while he was undergoing therapy. So, I suspect that while under treatment, he confided his memory, probably under hypnosis, to his therapist. If you remember the episode where she appeared, I think she indicated that she had broken up with the man she was seeing then, and PJ seemed to know him. I think that her boyfriend was Red John, and that he found her records of his sessions, and therefore knows things about PJ that he thinks he never told anyone. It's the only explanation, because PJ would never "forget" telling someone something like that, nor could anyone glean such specific information. It had to come during a time when he was under treatment, and probably under hypnosis. It's also telling that once again, last night's RJ accomplice was very matter-of-fact about her association with RJ, again appearing as if under some type of strong mind control. Excited to see how it plays out and keep an open mind -I don't think that RJ will turn out to be any of the seven that Patrick came up with, it's only natural that he would have excluded someone he might have met before beginning the chase of RJ at the CBI....