Haven't finished reading the posts so this issue may have been resolved, but only the first and second Hokage were from Senju... I think there may have been a reference to them as brothers. Third was Sarutobi sensei (with konohamaru and Asuma sensei) and the fourth's name was given... I don't remember what it was... but it twernt Senju. So no, Naruto is not of the Senju clan. The only confirmed Senju is Lady Tsunade and she has no children. As for the rest of the clan, as the dominate clan they were probably absorbed by the rest of the clans of Konoha. What I mean is, married off to other clans thus a large portion of the populous has Senju blood but not the name. Kind of like how only a few englishman could bear the name Tudor, but due to private liaison's many were related. This didn't happen to Uchiha because they were segregated from the rest of the village. Also, they never said Senju was stronger than Uchiha, it was only key members of the clan, most notably the first hokage who was strong enough to punk out Madara who would have us believe he was the first Uchiha to obtain Mangekyo. So imagine being on the battlefield and standing back and seeing your leader/mentor/protector beaten to a bloody pulp by one man... ya may back off a lil.
~~T~~