I do apologize for contributing to the derailment of the thread.
I'll try to get ahold of a black templars codex and check myself, but assuming BT characters work like everyone else's, the special vcharacters in the back of the book will basically take an HQ slot, just like a regular commander, chaplain, whatever. They have their own special powers, but they are more expensive, cannot be altered (It will say what, if any, wargear may be changed. Usually none) and sometimes require permission from the other player.
If a sopecial character can have a retinue, it;ll say in his entry. Special characters, regardless of their type, cannot be substituted for an upgrade character unless it syas speciffically.