Sachin,
Unfortunately, that is not possible. The only way to access an AdWords account data using the API is to use the client account's OAuth credentials to log in, i.e. for every new client, you would need to create new OAuth credentials. You can, however, use the same developer token every time, i.e. the one associated with your manager account.
Apologies for the confusion regarding the customer id. That should indeed be 10 digits.