A tip that helped me is to see the similarities with In and Out games and traditional grouping games. In a way, I&Os are just grouping games with two groups -- the In group and Out group. Animals in a zoo, objects on a shelf, students in a course, etc.

It probably depends on when you're planning to test. I personally would start at 51 (50 has an absurdly hard RC section), unless you are testing in April, then it might be better to start with more of the recent tests because you have a bit less time.