I never started out with sweet fruits. And kids really don't have to have those refined carbs of cereals at all for starters. I wanted my kids to get used to veggies first so they wouldn't refuse them, so we never did fruits first. Starting out with avacado is a really good fat and of course sweet potatoes and butternut squash and sweet peas. I only had one kid that was a fan of green beans over the peas. I agree the jar green beans are horrible!
Another thing is to cook them yourself. What I did was pick a veggie...lets say carrots and cooked them and then used a hand mixer (the little one) and blended them and then put them in ice trays....waited for them to freeze and then popped them out in a gallon size freezer ziploc bag. I did this with several veggies. You can get the butternut squash at Publix (I couldn't find it anywhere else...in the frozen section that is.) So, whenever the baby needed something, I just popped one into a little bowl and the microwave. They tasted so much better, is better for the baby, and cheaper. Whenever you have to go somewhere...really easy, just put a cube or two in a bowl with a lid or baggy.
Constipation is pretty normal with those cereals. I would lay off the cereals for a while and just do veggies and fruits.