Well, I think there's a few things that are for certain.  Obviously the William Blake theme is consistent throughout the show and the referencing can't just be a decoy.

A couple of facts:
1. We know that in the 7th episode of this season, we find out who red john is.
2. There are 7 suspects
3. The 7th episode is called "The Great Red Dragon"
4. "The Great Red Dragon" has 7 heads

A couple of thoughts:
1. Bruno Heller, with his love of theme and poetry & Blake, has purposely created the above situation.
2. Look up "The Great Red Dragon" on Wikipedia. The quote is right there for you all...

“And behold a great red dragon, having seven heads and ten horns, and seven crowns upon his heads. And his tail drew the third part of the stars of heaven, and did cast them to the earth.”

For me, and as a lot of other people have theorised, Red John is a 7-head collective of all of these men. I'm certain we'll find out specifics, but they're all involved and relevant. And all of them perform the different duties of Red John, and pose as him throughout the entire show. There may still be a 'master' Red John; but "the list is real" and all seven of them are a part of it.

All you people looking so ridiculously deep for tiny little fragments of coincidences are over-thinking it. I truly believe that Bruno Heller has left the series open for different endings and ideas. None of the hints are specific. He is Mar, thinking about the above was probably incomplete for "he is many".

Keep it simple.