Shimaichigai is a compilation of short stories/one-shot mangas/anthologies about Danchigai franchise. Each chapter is focused on one of the girls of the main manga (idk if the one-shots were written before Danchigai was created), containing ecchi and characters with completely different personalities.

1-2) Based on Mutsuki character.
3) Based on Yayoi character. Here she is a full-dere bro-con imouto instead of a tsundere, also Mutsuki is the tsundere here.
4-5) Each chapter based on the twins (I couldn't find out who was who), one is a nurse and the other one is the student council president. There are nudes, so proceed with caution.
6) Prequel of Danchigai, a short story about Mutsuki and Haruki, the first day he got into high school.

Haruki appears in all of the chapters as the Haruki of the Danchigai or another versions of him.

Overall: Worth it if you buy the volume and can read japanese, if not, just read the prequel that can be found barely easy. Finding the other chapters is a pain in the ass and took me hours, I had to ”read" it in a random thai website and I couldn't find chapters 2 and 4.