Should I use Core webservice API in a custom webservice API?

by Nguyen Nam.  

Dear all,

I am writing a custom web service (for using in the external app), with a list of steps, such as:

- Step 1; Create a new user (if does not exist)

- Step 2: Create a new course

- Step 3; Enrol the new course to new user with Teacher role

I know that, there are 3 threes core web services api (such as core_user_create_users...) corresponding to three above steps.

So my wondering here is: should I use these core web services api in my custom api or call directly to Moodle internal Lib file (such as /user/lib.php; /course/lib.php)?

My question looks silly, because I can not find an official document about Moodle internal Libs (in the opposite side, the document of Moodle external web service API is fully)

Thank you!