We have a single Bob and it is roomy- neither one of our babies fit very comfortably in it when they were young. We took the youngest to a parade when she was 5 months old and she kept falling to the side. (She is 10 months now and we went on a walk today and she loved it.) I'm not sure if other moms share this experience, but we always used the carseat when she was younger because of this.
Also, I personally love the convenience of leaving the baby in the carseat- it is so much easier to transfer a sleeping baby if you don't have to get them out of the harness and into the stroller.
You are going to LOVE the Bob- I wish I had a dualie!
Have you tried to fit the Chicco on the Bob? I don't remember the Graco fitting perfectly on the Bob or anything, we always used the red strap and the Bob conversion kit also came with a foam rectangular pad to put under the carseat at that time- don't know if it still does or if they fixed that. (We have a 2008 Bob I think...) I think you should try the carseat and see if you can get it to work before you buy a different carseat.