Given the rather diverse opinions being presented on optimal city placement, is it possible that there isn't one optimal position right now? I don't remember exactly which tiles in Scotland everyone is referring to, but I can think of a number of pros and cons for different placements as it is.
I've played a bunch of games as the vikings and colonizing Scotland is always tempting for me. Choice is often between settling one city in the middle or one east and one west. Settling one city gives it more resources to itself, but not all of them are reachable. Settling two cities gets more resources immediately, but fewer for each city. I think you can make a good argument for either.
As for on the iron or beside it, again there a few factors. I usually prefer to not settle on resources (though I guess that depends on the earlier discussion). Sometimes though, I do settle on the iron to ensure England can't get it without actually taking the city.
There are a lot of elements to this game. Sometimes there's a clear best choice. Often there isn't. And while I understand the appeal of making historically important cities more desirable (it drives me nuts when Constantinople isn't founded), part of the fun of this mod is seeing alternatives.