Bite my tongue
Let get to work. These are my views.
Most mayors will rule over Villages. Villages will start with 25 NPC that players can take over.
In the event that a village does not have enough NPC for all of the members of a guild to take over, then some of the members of the guild will have to start in other settlements.
If another member of your guild was to purchase a mayor package, he can merge it to your settlement, and thereby increase your settlement's starting land size by 9 parcels, starting npc population by 25 Citizens, and 9 parcel worth of buildings (Although these buildings are probably owned by the 25 new NPC Citizens that popped up in your settlement). This merge means that your settlement will now be able to accept more people from your guild at the beginning and they won't have to start in other mayor's settlements and take away their citizens (Since your guildies will probably run off from their settlement and join yours)
How does a guild from other games migrate into this game? You can probably consider guilds from other game as a settlement in this game. Since guild in this game is more of a "Blacksmith guild" sort of thing. You would be a settlement, while the settlement would then branch into guilds of different professions.
And no, you probably can't all start as part of the same family. The starting families won't have enough children to allow you to do that. It's eventually possible by all marrying to one another throughout the generations.