Here’s some mechanics I found while playing around with fuselages attached to side interstages:
Any fuselages that are detached will have no collisions with the part that the detacher is fixed to, regardless of part collision settings.
When you make a ring of detachers with vertical fuselages (think rocket style) which is elevated above the ground, when you detach the fuselages all of them will remain standing upright except for ones which are placed directly on cardinal directions, which will fall on their side. Not sure why, they just do that. (If a dev happens to see this, would you be able to explain why?)