Your post ended up in the "practice" section, so be sure future ones go into the main Forum.
The only important difference between the STI routine and any other subroutine is the interrupt mechanism. The same main routine watchdog applies (100 ms default) to a program using an STI as to one without it.
You can load up the STI subroutine with PID equations as much as the controller memory will take. There is no predefined limit on PIDs that any A-B controller will run.