First, study their characters. Know their personalities, backstories, and how they interact in the original story. For example, Shinobu is often calm but has a hidden fierceness, and Giyuu is a bit of a loner. Then, come up with an interesting plot. It could be about them going on a joint mission where they face a new type of demon. Keep in mind their relationship dynamics as you write.
Inosuke doesn't become a Hashira in the manga. The story progresses with him having other significant experiences and achievements instead of achieving that specific position.
It's hard to give an exact number. Different story arcs and battles led to the deaths of multiple hashira. Some died heroically, while others met their fate in unexpected ways. The count changes as the manga progresses.
Definitely not. Genya remains outside the ranks of Hashira throughout the manga. This is because of a combination of factors such as his skills, experiences, and the overall direction of the story.