Tummy Tuck Using Same Incision for C-section?

I was given a C-section in 1993 due to complications while giving birth to my son who weighed in at 9lbs 6oz. My problem is I was very a petite girl prior to becoming pregnant, which cause me to have a lot of stretch marks and LOOSE skin. Ever since my C-section- I am unable to keep a steady weight. And, regardless of my weight I always have a large amount of loose/fatty skin and an ugly scar to go with it.(and,the scar sorta has a chunk of skin missing in the center as well) My questions are; Can you re-open the same incision relating to my c-section? Also, is there a certain amount of skin allowed to be removed per patient per state regulations/standards? One last thing- I gained a huge amount of weight lately...(since I turn 32 yrs.old) Is it in my best interest to lose the weight prior to getting surgery?