As I see it, these aren't just ENT novels, they're novels about the history of the Federation. So filling in some family background for characters from later/earlier Trek series seems like fair game, if I can justify it. Tobin, as stated, is an established character from this period. We know the Paris family goes way back in Starfleet, hence Caroline. Commander Williams was named in honor of Shatner, suggesting to me the possibility that he was an ancestor of Kirk's. And so on.
Plus, let's face it, TOS is more popular than ENT. The Rise of the Federation
premise may draw in a number of readers who wouldn't otherwise pick up an ENT novel, so I feel it's appropriate to give the fans of other Trek series something they can recognize and connect to.
But it's a matter of taste. When I wrote the outline, I sent it to both Kevin Dilmore and Kirsten Beyer to see what they thought of it. Kevin loved all the continuity nods, while Kirsten thought they were a bit much.