There is so many 5thEd books, I am not certain if this has been answered in canon.
While at Durenmar after training Trianoma, Bonisagus studied magic from different wizards that Trianoma sent to him.
But were these wizards limited to only the other 11 Founders?
Basically I am thinking of adding the idea that there had been other wizards that Boniusagus studied, but for various reasons they never became Founders.
For example, I am thinking of the following 3:
Melusa, an ancient and powerful Fairy Doctor who was convinced to take the arduous journey from the Mediteranean coast to Durenmar and studied with Bonisagus for a single season before succumbing to the cold of German winter.
Alberich, a royal blooded sorceror of Merovech's line who was the first to study with Bonisagus. His Pride as a royal scion better than everyone else refused to let him accept Trianoma's plan to make all wizards equal, be they peasant or royal, so he left in disgust. Later on when the newly created Order of Hermes came calling with the Join or Die proposal, his pride did not let him accept the Join option.
Yu-Lian the Hidden, an exiled sorcerer from the distant eastern lands where silk came from tried to study with Bonisagus for many years, but was never able to learn enough Latin or other common language to properly explain his magic. Did manage to communicate that he was cursed to have his heart broken by his true love. Met his end by the mistake of ardently gazing upon the newly arrived Diedne while he was invisible, as Diedne demonstrated her spontaneous magic and flung a spear towards what appeared to be an empty spot.
Different SG's have diff interpretations of this period, and how "Hermetic Theory" (as opposed to "Blastum's theory" and "Thulsa Doom's Theory" and "Tim's Theory") came to be, but personally I think it's clear that he had to have sampled from more than just The Founders, even if not all those samples made it to the final format.
What he did was to try to find a GUT for magic, and to do so he would have studied as many different and disparate magics as he could.
It certainly makes sense that there were magicians who met with Bonisagus and even worked with him but eventually rejected the notion of joining an Order of Hermes. Nobody bats 1000. Some might even have learned the Parma before they quit. There must be some interesting stories about the fates of these wizards and their apprentices.
A lot of folk did - sorry if I overlooked your contribution in my linking. There was no objectively "right" or "wrong" answer there, so there was a lot of grist for the inspiration mill.
I think that one thing the big B had to do was survey a spectrum of approaches, of different personal or minor "theories", even if they were not particularly insightful or impressive. A wise researcher would recognize where there is nothing new to be learned, and also that every such example reinforces the (almost) universal scope of his own final product.