Yes, any weight of dumbbell can be used. This being said, if you have options, choose a weight that will provide you with a suitable challenge to see the best results.
While the Power Builder program doesn't have whole days dedicated to abs, it does include exercises that train the abs. Anything done in a plank position (push-ups, push-up holds, renegade rows, etc.) should be recruiting your abs. We use them for balancing work too. So while exercises like calf raises are, yes, primarily targeting your feet and calves, they recruit your abdominal muscles too. The "stretching" days of the program target the abs quite a bit too. The ab work is definitely in there!
If you want to do more ab work than what's provided for in the program, I would suggest checking out one of the "add-on" programs, such as
Back & Core,
Total Abs,
Core Strength, or
Code of Abs. These programs are designed to complement other, more whole-body focused work. They also tend to be fairly short workouts. So if you have a suitable training base already, you should be able to add one in without overtraining.