Lets say I have a squad of assault marines with a reclusiarch attached to it is the opposing player allowed to single out the IC instead of the whole unit
that's not strictly true. Wounds are dealt to UNITS engaged, and ICs behave as independent units in CC. So, if the combat looks like this:Nope. He must move to engage the closest model in the squad, and wounds are dealt to models engaged.
The z are your units that can't be harmed in CC, but can't attack either. If it were like this...o o ø ø ø o
x x x Y x x
z z z z z z
Your Reclusiarch would have been immune to CC, but could not attack.o o o o o o
x x x x x x
z z z Y z z