in my own experience, i was struggling (and changing and wasting money) when i was trying to fit the slider and get stable shots on a tripod.
Eventually (after 1 year or so with many tripods, heads and experimentation), i realised the contact point for the slider is the most important. You need to have as WIDE a contact point between the tripod and your slider for stable shots. I end up with a small tripod leg, but with the Manfrotto 701HDV for that reason.