In fact, it would be better to chose stretching over strength training if you had to make a choice between the two of them. Without stretching, your muscles become shorter, unbalanced, and inflexible just because of the tightening nature of strength training. Remember to stretch each muscle group you exercise and hold for at least 20 seconds. Make each stretch gentle, pushing yourself just until you feel a full stretch. Pushing your muscles into the pain zone will cause the muscles and tendons to reject the stretch and the risk of injury will increase.
Every exercise is an abs exercise.
This doesn't do much for your appearance or posture because I can almost guarantee you never perform that motion in your daily activities, unless you're lifting objects off the ground, lying on your back, using your forehead. All of your abdominal muscles will be activated and trained inward correctly. Connect proper ab contraction with other exercises and daily activities, and it will become second nature. Now you have a good idea of what it takes to safely make progress toward better health, so make a commitment to yourself to make it a part of your life's routine. Strength training works when you do it regularly because you're building a strong fitness base and adding to it every time you work out.