Actually, you're better off not boiling or freezing the greens--it's recommended because it makes the veggie in question sink, but it reduces the nutrition value for the food.
All manner of veggies can be fed. I feed zucchini, broccoli, spinach, and cauliflower. I leave the veggies in the tank for 3-5 days with no ill result, though they seldom last that long. Floaters can be sunk with rocks, veggie clips, or scewers. I don't chop anything up--the fish can tear of little bits, and the bigger fish leave shrapnel that the smaller fish clean up.
Peas are usually just for treating constipation.