1)I am not aware about your industry. In my opinion if you have hired a person as contract employee on retainership basis for specific period by signing the contract then there is no need to have a separate appointment letter. The terms and conditions maintain similar in the contract as per appointment letter.
-This can be utilised for highly qualified / Technical persons only.
-Don't adopt this practice for workmen or clerical staff category. Otherwise you will come in trouble under Contract labour abolition Act and ID act for their permanency.
2)If you are hiring a person through the contractor then there is no need even to have separate contract or appointment letter for each person.