For Error 1:
Replace
List<worker> stuffList = new List<worker>();
stuffList.Add(new worker() { Name = "John", Age = 23, Sex = true }); //error 1
With
List<worker> stuffList = new List<worker>();
stuffList.Add(new worker("John",23,true)); //error 1
For Error 2:
As you have created GetStuffList Method Parameter Same so u cannot do this in case of WCF Service as in WCF Service Method Overloading is not possible as c#.
You have to use Name Property of WCF OperationContract for that or you can make two methods with different name (
e.g GetStuffList,GetStuffEnumerable as per return type) and call as per your requirement in your code
For Error 3:
If you follow solution of error 2. I think Error 3 will also get it resolved
Happy Coding ...:)