Generally Haruka isn't too hard to get. There are quite a few people who split who need Reina and Sayumi taken. Riho, shockingly, is pretty easy to get most of the time.
Ayumi and Eripon? You need to be FAST. They go REALLY FAST and most splitters will already have Ayumi as their keep. If I had to guess who's the most popular in Idol!swap, I'd say it's Eripon and Ayumi.