I play on Gaheris as a Firbolg. Can I have merchants from the other two realms in my house?
Yes. On Gaheris and Mordred, you can place NPCs from any realm in your house. Realm specific mechants must be purchased from vendors in the marketplace of the realm you wish the NPCs to be from. For example, if you want a Midgard healer NPC for your Albion style house, you must buy it from the merchant in the Midgard housing zone.