the slipped stitches along the gusset edge are there to make it easy to pick up the stitches
pick up a stitch using both thread of the "chain"
pick up a stitch using only 1 thread of the "chain"
the picked up stitches are often loose (--you really have to think about making them tight, or they are always too loose!, (even i have to think about it!)
to resolve this, you can twist the first row of stitches as you knit them.
Occationally, you make a real mistake, and pick up X on one side of gusset, and X+1 (or X-1) (where X is the number the instructions said to pick up)
don't worry, just decrease to right number --one gusset will have 1 extra decrease, (or you can hid the extra stitch in a decrease in center of 'heel/sole')
(the only one who will know will be you!)