Does it violate any normalization rules if I add a boolean column to a db table?

Database tables:

Servers

ServerType

[dbo].[Servers].[ServerTypeId] is a foreign key to the column [dbo].[ServerTypes].[Id]

I have a requirement to set the priority server for a serverType.

So I added a column IsPriority to Servers:

ALTER TABLE [dbo].[Servers]
ADD IsPriority BIT NOT NULL DEFAULT 0;

Does that violate any normalization rules?

Should I create a new table PriorityServers and add relations to the Server and ServerType table?

Techincally speaking, proper database normalization would ask for a separate table. But enforcing uniqueness would get complicated, requiring two unique keys and two foreign keys.

Your existing solution is much easier. You just need to enforce that only one Server of each ServerType can exist at once. You can do this using a filtered unique index.

CREATE UNIQUE INDEX UQ_Server_ServerType ON dbo.Server
  (ServerType) INCLUDE (Id)
WHERE (IsPriority = 1);