All guild members can port to any of the guild buildings.
Re: Houses - Cottages are mainly cosmetic, but you can put chests and beds in them, as well as personal crafting stations. Farmhouses, medium homes, and larger homes can be upgraded into specialized structures - Farmhouses turn into Harvester, Mining and Rancher, and manors and above can be turned into Trade, Apothecary, or Armorer.
Farmhouse conversion can be profitable of you take full advantage of the feature. I should have on up this week in the valley, while one of my officers has two on the ridgelines next to the valley. Feel free to check them out. Ask away - still waiting to find you in game - lol.