reverse slider: Drag the end of the slider on the timeline. Note that your cursor needs to be pretty accurately in the middle of the sliderend, although the cursor will change to a double arrow when it's in the right spot so it's easy to find
making a shorter/longer slider with the same length on the timeline: Press F6, click the green plus sign in the window that pops up to make a new inherited timing point, and change the slider velocity on the left side of said window. It may take a few tries to find a number that has the effect you want. Also note that you'll need to add another inherited timing point like above to change the slider velocity back to normal if you only want one or a few sliders to be different.