After hours of research, I look to you all, because I cannot seem to find a solid one way or the other

. My questions is whether
ICs from different detachments of the same faction can join another detachment. For example: Det 1 has a Warboss and two units of boys. Det 2 has a Painboy and two units of grots. Could the Painboy join one of the units of boys? Another example: the formation Ghazghkull's Bully Boys from WAAAGH! Ghazghkull includes 3 units of
MANZ. Could a mega armoured warboss, or even The Big Boss himself join one of those units, even though it would be a separate detachment?
Like I mentioned, I cannot find anywhere that says no, but nor can I find anything that says yes. Your interpretations would be greatly appreciated! Thanks!