Re Clans to play:

The Prima Strategy Guide lists the following difficultly levels for the Clans:

Mori ( Red ) : Medium
Shimazu ( Green ) : Easy
Imagawa ( Mid Blue ) : Medium
Oda ( Gold ) : Hard
Takeda ( Black ) : Medium
Hojo ( Purple ) : Medium
Uesugi ( Dark Blue ) : Hard

I have played and won ( @ Difficulty = Normal ) with : Mori, Imagawa, Takeda, Hojo & Uesugi.
I found Mori & Hojo the easier of the medium ones ( and Uesugi to be easier than Takeda ! ).
In my opinion, you should be happy with your playing standard before moving onto trying the "split" clans of Imagawa & Takeda.

Enjoy.