RBT's single shots in relation to Skirmishers

[Dragon fire]

How does the RBT piercing ranks work when I shoot against skirmishers. As such they are described in the BRB as having no rank. Does that mean than a single bolt does not pierce ranks? A small additional question to this is how this works in relation the extra damage from the flamespyre calculated from the wake of fire?

Secondly. How is an RBT single bolt against the flank of a unit of skirmishers handled if there is 5 skirmishers + 4 characters in the two ranks. front rank 4 characters 1 skirmisher, second rank, 4 skirmishers. Are the first skirmisher killed thereafter I hit characters or does it randomize? Second part of the question is if it is the same if there were only 4 remaining skirmishers?

[Asurion Whitestar]

Skirmishers have no rank in combat (only).

A shooting attack needs to target the unit when it has less than 5 rnf to be allocated to characters.
The Bolt Thrower shot will not hit the characters in the unit, unless the unit has 4 or less models.