We generally don't know what compilers the customers were using. The key thing is that you will need to have full access to all of the Ada source code, including that supplied by the compiler vendor, unless it is part of the "standard" Ada library.
The functions starting with “INT_” are functions internal to the OS Abstractor, In order to keep the application portable, application should not use those functions (use only the functions that are exposed in the OS Abstractor manual).
In Ada, the bounds of an array (the 'First and 'Last) are generally passed as a separate argument. This argument is traditionally called the array "dope" or array "dope vector" (e.g. see https://en.wikipedia.org/wiki/Dope_vector).