Why not use Oviedo (which is the tile where the mountain used to be in the north-west of the iberian peninsula) as the first spanish capital? After all, the kingdom of Asturias was THE FIRST christian kingdom in the iberian peninsula, its a good spot for a city, it has access to two clam, fish, cow (or was it sheep?) and iron. We could give the moors only one city, granada, in the spot where sevilla would normally be (right above the hill that is at the very bottom of the peninsula, this is where they managed to remain historically, until the christian kings finally decided to push them out of spain, taking granada).
This could fullfill the purpose, for historical accuracy, of ACTUALLY building "la mezquita" in southern spain which is pretty tough to do right now as the arabs. As for Madrid, Madrid wasn't the spanish capital until king phillip II decided to make it into the capital in the 16th century (thats 800+ years after the spanish spawn), before that sevilla (which is where we will place the moorish city, except the moors could call it granada, or cordoba) was the capital. The player could still build madrid if so wished, but it shouldn't be a historical requirement as it became the capital pretty late imo.
For example, Istanbul is not the capital of turkey anymore, although it was for quite a while, and it is still represented in-game as staying as the capital. Before Phillip II showed up, seville was not only the capital, but also managed the colonies, even after the court was moved to madrid, sevilla still managed the colonies although it was controlled by madrid. I am suggesting, for gameplay purposes, that the capital be moved to sevilla once conquered from the moors and not to madrid. In my experience playing spain, the best way to fully use Iberia is with 3 mega-cities, where none steals tiles from the other, those 3 cities are Oviedo, Sevilla and Barcelona, Madrid's position in the center would make it "leech" from the other cities, madrid might be good in a bigger map, but not in the one in RFC.
As for barcelona, I think it's best that it is simply founded by spain, as usual, rather than introducing an independent civ or conquered by the moors, let's just keep it simple giving the moors the 1 southern city I explained and have spain spawn with 2 settlers perhaps, which should be used to found, besides obviously Oviedo, ideally barcelona (if the human player, too avoid being too deterministic, the AI could found Bilbao, Valencia or another city once in a while.)
I am not sure if my ideas will come across as too extreme, but I feel what I suggested is best for gaming purposes. My main frustration was not being able to build la mezquita in southern spain. The only other problem I see is that la mezquita requires islam, so if the spanish conquer the moorish city they will keep the building (which they did IRL, it was simply converted to a catholic church), but Islam will stay there, IRL all muslims and jews where expelled from spain. To solve this, maybe we could have a one-off exception where spain can remove islam from the moorish city, yet keep the la mezquita wonder or an event to the same effect or give them an inquisitor ability like in rfc europe (although the last option could be unfair as spain already has a sea-related UP).
All in all, I don't think Iberia should be too complex, as there really should only be 3 cities (until portugal shows up that is, but spain doesn't settle in portugal anyway, neither should the moors, to keep things simple.)