I've always found this era interesting to speculate on but it came up again the other day when I was thinking about Mercere magi at the start of the 9th century, prior to the formal admission of Redcaps at the Grand Tribunal of 817 per the HoH:True Lineages timeline...
For instance if the Order is founded in 767:
Mercere loses his Gift in the Winter of 773 and after an initial period of denial and desperation, disappears for "more than a decade."
His only 2 Gifted apprentices, Priamitus and Hermes Triceres / Mutant who are also his sons, are both noted to have not finished their apprenticeship when he loses his Gift Both finish their 15 year apprenticeships through self-study and the tutelage of other magi who took an interest (probably followers of Bonisagus/Trianoma), say by about 780 or so. Mercere takes no other Gifted followers, forfeiting all other potential apprentices to Houses Bonisagus.
Priamitus may be in his early 30s at this stage and begins to reform a version of the Cult of Mercury primarily with the help of House Flambeau.
Mutant is probably early 20s at this point and becomes a metamagic theorist (probably with close links to House Bonisagus) and is noted to have little interaction with his father or the House until the birth of his Gifted great-grandson which is probably not until after Mercere himself dies in 818 even with the suggested fecundity of the House.
Mercere reemerges in about 885 or thereabouts and calmly begins to adopt unGifted apprentices and tutoring them for 15 years.
At the first Grand Tribunal of 799 then there are only 2 or perhaps 3 Gifted Mercere magi - 1) Priamitus, who is unlikely to have completed the training of more more than one Mercurian apprentice and 2) Mutant whose ultimate first apprentice his great-grandson is likely no more than a twinkle in his father's eye...
Mercere has lost the Gift and his first unGifted apprentices may have only just be finishing their unconventional apprenticeship.
This is perhaps not that dissimilar to the modern House in some ways, but by the time of the 2nd Grand Tribunal in 817 there are only a handful of Mercurian Mercere magi (associated with a larger group of lineages within House Flambeau that form the nucleus of the reformed Cult of Mercury) and perhaps the beginnings of the Mutantes lineage.
It means that I can't really have an NPC Mercere magus as a founding magus of my conception of Urbs Rubra easily as it was founded in 802! (damn)
It also means that House Flambeau would partly consist of an odd collection of almost Ex-Misc style Mercurian magi and some Mithraic cultists as opposed to the more recognised military types of later conceptions.
I just thought this was interesting to develop.