I like a separate session for abs. This can be done at home with just a little equipment.
Also, you need to re-think your rep strategy. Here’s a cut and paste from an older T-mag article called ‘evolution of ab training’ issue 96 -
"Poliquin was one of the first training gurus to popularize low-rep ab work. The theory is simple. Slow-twitch fibers respond best to higher reps and low resistance. Fast-twitch fibers respond best to heavy resistance and, consequently, low reps.
You probably knew that, but did you know that the abdominals are composed primarily of fast-twitch fibers? In other words, if you’re one of those “three sets of 100 crunches” types, then you’ve been doing it wrong or, at least, not optimally.
According to Charles, performing more than 15 reps per set for abs will probably reduce the effectiveness of the exercise. Instead, do three or four sets of 8-12 reps, using some type of resistance and a slower tempo that emphasizes the eccentric phase of the movement. If you’re more advanced, you may want to perform as many as ten sets for abs, but keep the reps in the range of four to five.
Think about it, would you do 50-rep sets of biceps training? No, yet many people think nothing of getting on the floor and crunching their day away. This high-rep philosophy is probably left over from the spot-reducing myth. Remember Arnold talking about “whittling away” the fat with high reps? He was wrong—big, cut, and able to kick Satan’s ass, but still wrong.
The trick here is twofold. First, you’ll have to use resistance. Second, you’ll need to use a full range of motion. While the Swiss ball has run into some recent criticism, it’s still probably the best tool for ab training. The shape allows you to get a full range of motion, and the instability helps improve your balance, a must if you’re an athlete trying to approve agility.
The two best Swiss ball movements are Swiss ball crunches and reverse crunches. Allow the shape of the ball to give you a good stretch at the bottom of the movement. Start with three sets of 10-12 reps, and work your way to ten sets of four heavy reps. Hold a dumbbell across the upper pecs for resistance. If you must, wrap it in a towel for comfort. Yes, people will call you a big girl, but ten sets with a 60-pound dumbbell across your chest can get uncomfortable."