Yes, if you do ISO you will do it in P-cola. As for tests, 80% is considered marginal. As in, you pass, but you need to shape up. Sometimes instructors have students who score an 80 or 82 percent remediate. Sometimes they don't. As for the hardest of tests, most are relatively easy as long as you bust your butt and follow the study guidelines laid down in the day 0 brief. Remember you're there to get wings, not drink your butt off. Navigation has the highest failure rate; the standards are very strict and the multiple choice wrong answers include common mistakes, not just random numbers. So you really need to have the process down for Nav before you take the test to do well.
As for ISO, to my knowledge you now have to apply for the program. As the aviation officer at your unit for more info.