It doesn't replace anything, if thats a worry. You can open and close its tab group as you can with layers, navigator and such tabs.(If that made sense) You can always uninstall it if you don't like it. Its a free extension, so you really have nothing to lose.
Remember that once you install it, before you can use it do the following: In photoshop, click File>scripts>NKS5_INSTALL. Then it'll work. Otherwise it just wont actually select its tools if you click them. I don't understand why, but thats what I had to do when I got it.
I dont know any specific brushes, but I kinda have this problem too. What you can do is, find a brush that is close to the look/feel you want, and just lower the opacity if its too strong. You may be able to adjust the hardness of the brush as well that was you can match it up with the sensitivity you prefer.
I dunno. I've tried a good number of photoshop's own brushes and none of them feel very friendly for sketching regardless. They all feel too stiff or rigid for my liking. I did try SAI a while back and it felt more fluid for doodling. Catch is I use Mac. Apparently Japan can't be bothered to try macosx