Harder difficulties have more notes, but they're more difficult (obviously).
To get some love gems and money, idolize and max bond one each of the lowest-value girls that you wouldn't normally put on your team. Feed her to herself to make her wear the costume. Put one each of a certain color on a team. Edit: meaning one of each girl, but all from the same color so you're not throwing away score points. You have to get at least a C ranking in score to get the food.
Play the song with the longest combo that you can reliably achieve. The bond points will be divvied up amongst the girls. Girls with maximum bond will generously give their share to anyone else who needs it.
When an idolized girl gets max bond, click through her "story" and you never have to use her again. Don't forget to collect the album rewards for exploiting 5 girls, 10 girls, etc.