No castmates for any of them unfortunately, but thebastion has all of your other preferences. Part of the game involves exploring hostile territory for survivors and resources, so your characters would definitely have something to do in-between events.

Also, for your Shepard, the game is small and is lacking a PC leader of sorts, so Shepard would definitely fit right in. I may possibly be able to enable a Garrus if you app Shepard as well, but no promises there.