There are plenty of reasons why it's totally plausible that Brett Partridge is Red John. He fits the description given by Rosalind Harker. He's a forensics expert. He "irks" Patrick Jane. His voice has that same odd, soft tone to it. We all know this.
I think there's a lot of multi-layered meaning in the characters' names, particularly those of the seven Red John suspects. The name "Brett Partridge" is one of the most interesting. According to Greek mythology, the first partridge appeared when Daedalus threw his nephew Perdix off the sacred hill of Minerva in a fit of jealous rage. Perdix, as an apt scholar under Daedalus's charge, showed considerable evidence of ingenuity. Walking along the seashore, he picked up the spine of a fish. Imitating it, he took a piece of iron and notched it along the edge, thus inventing the saw. He then put two pieces of iron together with a rivet at one end, sharpened the other ends, and thus made a pair of compasses. (Yes, the myth specifically says iron. Possible origin for the Tagliaferro alias?) Daedalus could not bear the idea of a rival, and was so envious of his nephew's accomplishments that he took the opportunity to push him off a high tower atop the hill. Athena, who favors ingenuity, saw Perdix falling and changed him into a bird -- a partridge. Mindful of his fall, the partridge does not build its nests in the trees or take lofty flights, and avoids high places. For his crime, Daedalus was tried and banished. Athena brands him with a partridge so he will never forget the crime.
To me, this could mean several things in regards to Brett Partridge/Red John:
1. As I said, it might serve to explain where Red John got the name Tagliaferro. Yes, Tagliaferro means "cut iron," but why pick that name? It's an awfully specific and uncommon name to pull out of thin air, which I doubt Red John would do anyway. Remember, he considers himself a showman. Everything is intentional, and there are no coincidences.
2. We already know that Red John is/was somehow involved with Visualize. We know that Timothy Farragut, the original Visualize guru, was quite possibly murdered by Bret Stiles, and we assume that Stiles would have done this purely for the sake of power. But what if there's more to that story? I don't think it's a coincidence that Stiles and Partridge are both named Bret/Brett. The writers of The Mentalist could have picked any name, so why give the same name to two characters, especially when both of those characters turn out to be such significant parts of the story?
My own personal theory, based on the Greek partridge myth -- Bret Stiles is Brett Partridge's father, or the two are at least related. As the guru of Visualize, Timothy Farragut would have been something of a "teacher" to all his followers, including Bret and Brett. Farragut observes that despite his young age, Brett has gathered a number of his own followers within the cult. Brett is a skilled manipulator capable of great charisma, a worthy rival that Farragut feels compelled to somehow rid himself of. Farragut's plans, whatever they were, go awry, and although he manages to cause Brett's "fall from grace" within Visualize, he is "banished" (killed) by either Bret or Brett in 1977.
3. Just like partridges avoid literal high places, Red John avoids figurative high places in society (prestigious jobs, appearing to be particularly gifted at his job in forensics, or anything that would call too much attention to his true identity) as Brett Partridge. He is mindful of his prior fall, and he lays low. Every other suspect on Patrick's list has a fairly public and relatively high-ranking job, whereas Partridge appears so average that he just kind of disappears into the background of society. If you want to be a notorious serial killer, you have to appear completely mediocre.
I do think the other six people on the list are associates/followers of Red John, and that Gale Bertram is Red John's right-hand man. I think the name "Bertram" is a reference to Prince Bertram the Bad, a children's book about a prince who was "born bad" and enjoys terrorizing the people around him. It's an obscure reference, but all the illustrations in the book are done primarily in red tones (even the cover is red). I don't think that kind of imagery is an accident.
