Currently, as far as I know, can only have the mesh mirror from left to right. If you happened to have done your modeling on the other side and want to mirror it over, you have to scroll all the way down to Tool > Deformations and use Mirror, then perform the Mirror and Weld.
A little checkbox next to Mirror and Weld would be great.
You have that option already. If you look on the right of the button (actually on the button) you will see a small X Y and Z. Click the axis you want. (This is the same method as used elsewhere in ZBrush such as the Deformation menu.)
I know, but you can not say you want to go negative x for example. It will always mirror from left to right, if I recall the help correctly.
An option to mirror from right to left would be great. At least there is the workaround via Deformation > Mirror. But it’s not an elegant solution.