M.B.
Here is one point...
different types of potatoes cook differently...
The russets, or "baking" potatoes, get mushy... think of what they are like when you bake them..... they will break apart easily when stirred...
Red potatoes are "waxy" type of potatoes... they hold their shape much better.
Yukon golds are a nice blend of the two.... they are firmer than russets, but not as firm as reds....
http://allrecipes.com/howto/perfect-potato-salad/
This article also talks about what kind of potato to use.....