I need a regular interrupt but the actual rate is not important. Either the millis or servo rate will do fine. Can I add a few lines that will execute when either of these existing interrupts fires?
Why are you using interupts? Unless you need to do things on a microsecond timescale, you’re better off not using them. See the ‘Blink without delay’ example in the IDE. Go File > Examples > 02.Digital > ‘blinkwithoutdelay’.