So after reading the prologue to AMoL, I am even more convinced that Taim is in fact the false dragon formerly known as Guaire Amalason and I believe from the very beginning was a protege of Ishamael. Amalason was the most powerful of the false dragons, and it was he who during the Second War of the Dragon conquered virtually all of the westlands with the exception of Tear and Tar Valon. More is known about Amalason and his actions during the Second War of the Dragon, than any of the lives of the other false dragons. It is my belief that Amalason was a darkfriend and false dragon set up and mentored by Ishamael and is in fact presntly known as Mazrim Taim. Lets go through my rational.
- knowledge - Taim has an advanced knowledge of weaving Saidin, much more so than someone with no master to teach him would be expected to know. As a result of the prologue we can deduct, that Taim knows that by channeling through 13 myrddraal, anyone can be turned evil. This is information that even some Aes Sedai aren't aware of. Likewise, Amalason was skilled in the use of Saidin, killing and stilling several Aes Sedai, he was also someone who was said to have in depth knowledge of the Prophecies of the Dragon (WoT wiki) possibly gaining this knowledge by Ishamael.
- language usage- When referring to the Aiel, he calls them "the so called Aiel" making a reference to the days when the Aiel were pacifists. Knowledge that only clan chiefs, wise ones, and forsaken are privy to. He also does not use the word stilling when refer to a man being cut off from Saidin, but rather he uses the word "severing" a word only used in all of WoT by none but the forsaken. From these and other examples we can deduce that Taim is like the forsaken, very old himself, or has spent extensive time around a forsaken who uses ancient vocabulary.
-Time - This is the tricky part bc Ishamael was at times bound in Shayol Ghoul and other times loose on the earth, so for my theory to be correct, Ishamael would need to have been loose at the same time Guaire Amalason lived and came to power. Luckily, we can deduce that in fact he was. In the War of the Second Dragon, it was Artur Hawkwing that finally defeated Amalason and we later learn Hawkwing's High Councilor Jalwin Moerad was in fact none other than Ishamael in disguise. It was he that poisoned Hawkwing against the Aes Sedai. The forasken love to scheme and it is next to impossible to believe that if Ishamael was loose at that time, that he played no part in Amalason bringing chaos to the world and conquering nearly all of the westlands. Taim also has not gone mad from channeling Saidin, given that he has been channeling for some time according to him, the only way he wouldn't have gone mad was if he had the dark ones protection.
-miraculous rescue- After Taim was captured by Aes Sedai, his Aes Sedai captors were mysteriously killed and Taim escaped. We can therefore conclude that he was already a dark friend having received aid from some powerful individual that was able to overcome his six Aes Sedai captors. I believe a similar situation occurred when Amalason was captured and brought to the White Tower. As Amalason's and Hawkwing's troops fought in the streets of Tar Valon, Amalason was rescued by Ishamael in the chaos and therefore not stilled. Ishamael has a history of rescuing those under his command, having rescued Lanfear for example.
-age- after being rescued by Ishamael, I believe that Amalason was put into a stasis box until Ishamael thought it prudent to release him. It is well known that he doesn't take failure lightly. Only this could explain Taim's current youthful appearance because even though channeling can slow the process, not to the point of the thousand year in between Hawkwing's time and AMoL. It is plausible that Ishamael had access to a stasis box, because in Towers of Midnight there is a scene where Graendal is taken to a room that is literally filled with Ter'angreal and Angreal and given a dream spike. It is plausible that during the periods in which he was loose, Ishamael acquired a stasis box.
-clothing- Taim is obviously Moridin's follower. Moridin is the one who introduces him the rest of the forsaken. Taim always wear's black and red (Moridin's colors), it is also the colors that Moridin forces Cyndane and Moghedien to wear in their servitude. On Taim's sleeves are dragons in mocking of the dragon reborn.
Taim hates Rand, but why? Because Rand's success reminds him of what almost once was. Amalason conquered nearly all of the westlands and if it weren't for Hawkwing, he may have very well succeeded in defeating Tar Valon and declaring himself the dragon. All-in-all all these combined lead me to believe Taim and Amalason are one. If you read a WoT with these things in mind, you might just be surprised by what you find.