Write a MongoDB code to insert several documents into the collection named Employees. Also, make the data schemaless by adding gender to the second document. The documents should be the following:
{ "Employee" : "Tedd" },
{ "Salary" : 15000 },
{ "Employee" : "Robin" },
{ "Gender" : "Female" },
{ "Salary" : 2000 } ,
{ "Employee" : "Barney" },
{ "Salary" : 100000 }