Correction Only one person has been SHOWN to use the Mokuton."It is very possible that the Senju clan disbanded into smaller clans and families, especially since they never had specific traits or techniques like the Uchiha or Hyūga clans with which to easily identify them."
And only one person has been able to use Mokuton.
Also how else would they be easily identified? Tobirama looked completely different to Hashirama and didn't possess Mokuton but mastered Suiton? Maybe you need to be an expert in both as a Senju and you can then use it.
Because they are Kage level they have huge chakra reserves and she is an expert in chakra control due to being the most renown Med ninIt's just an assumption I guess because their kages and very strong. But in Tsunades case she has shown to be very strong in body and chakra esp when she distributed her slugs to all the konoha citizens and protected them.