I think that Ultramarines are supposed to be the "vanilla" Space Marines, ie. they have all the Space Marines units, while the other chapters get "Unique Units" so to speak.
but saying that, i guess they do need a unique units for themselves, right?
EDIT: The only "UU" the Ultramarines have in the tabletop game (according to the UK 40K site) is the Tyranid Hunter, which i guess is kinda the same as what you said, right?