The domus magna of the Mercere, Harco, is in the Roman Tribunal - home to a large number of seafaring mercantile city states (technically all republics, i.e. entities ruled not by a single lord, but by an elected council).
In 1220 the most powerful and famous is probably Venice, mystically wedded to the Sea, and self-proclaimed "ruler of 3/8ths of the world" after diverting the 4th crusade to do a lot of dirty work for it. But Genoa, on the other side of the Italian Peninsula, is a close second, and a fierce rival. Then there's Pisa, that will shortly succumb to Geona's might. And there's oft forgotten Ancona, in the Adriatic, scourge of the Saracen pirates.
With the exception of Venice, whose first Doge was elected in 697 (but becomes a real power only after the crowning of Charlemagne in 800) they have all established themselves as seafaring city states after 800 (in the same period, a few more like Amalfi have managed to rise and fall) despite some of the corresponding cities predating the Order by over 1000 years. In other words, they seem to exist in the same period and in the same area as House Mercere, they have the same focus on mercantilism, and they have this peculiar "democratic" organization that is reminescent both of the Order and of early Rome, and is easier Code-wise to work with than a lord's fief.
So it's natural to ask: is there a connection between them and house Mercere? Maybe different branches of the Mercere "family" are each an established, powerful family in one of the mercantile city states so as to hide house Mercere vast economic might to the world at large? Or perhaps there is some other mystical force behind their birth and prosperity ... and the same force has molded Harco and by extension House Mercere into what it is now?
Connection, probably. Direct control? Unlikely. Mercere works out of a small town at the base of the Italian Alps, but likely has individual Mercere in each city running mercantile concerns which are also used for message running. Thye may well have led the way against the church prohibitions against banking and profitable trading, inspiring others in the city.
Even without direct control, technology tends to bleed out into surrounding society. Hermetic democracy and banking must have bled out, at least a little. House Meecere is, by Hermetic standards, wildly exogamous and prolific.
I could see the ideas of House Mercere bleeding out in quite specific ways, from individuals, accidentally. There is no need for direct teaching, just competition between merchants, and copying success. It would probably work both ways.
The Florentines will become masters of information, in the near future, based on their familial correspondence and information networks. That sounds like something learned from a Redcap family. Redcaps are usually literate, widely travelled by definition, and information carriers.
Genoa is right on the coast, and a perfect departure point for a Redcap trading expedition. Carrying low bulk high value merchandise is a Redcap specialty (vis). And the Genovese focus on high value, low bulk trade (spices, slaves, etc). What a coincidence!
So yes, it could easily be included in a saga, if commerce was of interest.
I have always considered House Mercere to be their own semi-secret mercantile republic. While they might be able to influence the big city ones, they would not want to have any control. Doing that would lead to a big risk of allowing outside (non-family/blood) to potentially access their exclusive customer base which is one of the wealthiest and most powerful in ME.
Run them like a merchant insurgency. While they might own and control a massive collection of small trade operations, mundanes would not know that they are all held by extended members of the same family. With their access to magic communication and travel, outside entities would have almost no chance of ever figuring out they are all connected and controlled by a single group.
I know it's bad form to point out typos, but I love the idea of House Mercere having Recaps.
Clearly, it's quite a junior role: they travel round to covenants with a Redcap, and before the Redcap speaks, they remind the covenant of everything they were told on the previous visit.