I would say Sneak is all important, Onehanded or Bow is all important. Some Alchemy to make poison.
You need to be able to sneak and do damage.
I would lean on those skills early; kind of like starting as an 'Asssasin', and Easing into 'Thief' later.
Once you are a stealthy killing machine you can worry about improving up other skills.
If you have too many skills you may find it difficult to kill things, as your combined skill increases your level (and the enemies level)
You kinda want your stealth and one hand weapon to be as high as possible early on, compared to your other skills for that reason.
That is what I would do for a Thief/Assasin.
If the above is not good advice, don't worry; someone will be along to correct it