[Draconis Argentus Falzone]

Alright. To settle some points.

In a Space Marine army, a Commander may lead a Command Squad, as may a Librarian or a Chaplain.

In addition, you may attach a Librarian and/or a Chaplain to a command squad, but may not take two of the same type within a single command squad, nor may two librarians be attached or two chaplains.

In short, you may have the following as a command squad, and thus as a single HQ choice:

Commander (of either rank) - Librarian (of either rank) - Chaplain (of either rank)

Librarian (higher rank) - Librarian (lower rank) - Chaplain (of either rank)

Chaplain (higher rank) - Chaplain (lower rank) - Librarian (of either rank)

Each of those three combinations of ICs is actually one HQ choice. Although it's just as expensive as it is broken, so the nerfing equalizes the cheese.

As to taking bikes. I looked it up.

In a command squad, i think you have the following:
Sergeant (who can take wargear) and 3-9 marines. But I'm not sure if the min is 3 or 4...

Anyways. IF you can have a min of 3 plus the sergeant, then yes. You can buy the entire command squad bikes or jump packs provided you keep it to a total of the ICs, the sergeant and the three specialists... But I almost think the minimum is 4 (don't have the codex right with). If the minimum is 4, then, no, you can't...

Also, check the wargear descriptions, I would almost place money on the possibility that there's some little rule saying bikes and jump packs may only be taken by independent characters...

Edit: Friend of mine confirmed, minimum is 4. Also, Commanders may lead and other HQ may be attached, but since they did not join at the start or during the game, they also may not leave the squad. They are "attached" after all...