1/4in seam allowance should be fine for normal wear, especially in a loose fitting skirt (as in the photo). I sew professionally and all of the patterns I design or modify use 1/4in SAs where possible - with outlets to allow for alterations or seam finishing techniques. If it was serged and has been through the wash a few times I would worry about pre-existing damage to the fabric from the double needle serger seam. An amateur unpicking a serged seam is also likely to do some damage. Maybe plan to take up 1/8 extra inside of the existing seam line.
You might want to add a zig zag or mock-overlock stitch to prevent raveling, but they should never be used as the construction seam. Use a needle appropriate to the fabric, and if it is a high-stress area consider a triple stitch, although that may add bulk. Avoid adding an in-seam pocket in an area that falls just over the hip - drop it a little lower or the pocket will tend to gap open and make that area seem wider.
It should also be pointed out, this type of pocket modification should really only be used for wovens where the seam is on or close to the straight of grain, it would stretch out of shape on most knits, or if the seam were cut on the bias.
