While they may not be joined during the creation of the squad nothing stops them from joining once the game starts. In theory you could create a full command squad with Master, Libby, Chappy, then join an extra HQ character say a second Libby, then you could also join 3 stand alone Techmarines to it. In theory there is no limit to the number of characters you can join to the unit.. the limit is they have to join a unit and cannot just join together. What Im really asking is who think its "beardy" to join multiple characters to the same unit outside of the standard command squad? If you think its beardy please tell me why.
Example units.... Master and Libby taken as stand alone HQ slots then joined to an assualt terminator or veteran squad.... 2 Shas'O Crisis Suits joined to a full Elite Crisis Suit Squad