I wouldn't worry about Sam taking over Hawai'i or stealing the spotlight from one of their other regulars just yet. The initial reports indicated he'd only be a recurring role, though admittedly that's being somewhat muddled by further reports since that say he's "joining" the Hawai'i team, though those two terms are not mutually exclusive.
As for potentially overshadowing the show's cast, it should be noted that during late season 2 they did set up the storyline with Tennant's son being accepted into the Naval Academy. Assuming he does go off there in the third season, that does create a vacancy in the show's main cast which should make it possible to accommodate Sam without having to neglect anyone else in the cast.
And finally, there's
this article in which Vanessa Lachey voices her enthusiasm over having LL Cool J joining the show which certainly does not sound like she considers him a threat to her position as the show's lead. She also seems to indicate this decision was influenced by the success of the franchise's three-way crossover this past winter.
I'm not trying to dismiss your concerns entirely, I'll admit I have my own reservations about the idea. But I trust the show's writing and production staff enough that I'm willing to give things the benefit of the doubt and see how things play out.