Pilates is tough. I would say don’t do it if you do not already have a lot of strength in your back and stomach muscles. Pilates basically works the same muscles as ballet, and most other exercise is very different. Much more inner thigh strength is needed, many positions your legs are turned out. I would say no harm in trying, but if something is difficult don’t push yourself, modify or skip it.
Using weight machines, not free weights, will work specific muscles, and is better at making sure you don’t injure other areas of your body. Although, for posture I recommend dance or aerobics more than anything, especially dance or a zumba class.
Lastly, if you have an S curve, be sure to wear your purse on your low shoulder.