Are Momiji/Ayane/Rachel playable in the main story?

[Somersault Justice]

Or is it just a couple stages and that side mode like in Sigma?

[Akimbo]

Same, i'd like to know if you can select chapters and/or select what character i'd like to play from the menu

[Dr.Scrumptious]

They have their own dedicated missions placed throughout the Main Campaign.

[Atom]

It's just like Sigma 2 as far as I can tell, so 1 short stage each in the story/chapter challenge, and then you can use them in Trials but not Tag Races because that feature was cut.

[Arbor]

There’s a tag mode that replaces coop