I think if you are adding butter you should make sure it doesn't have salt added. I think 'can't believe its not butter' would have salt and other additives.
I've been told ot add fat to veggies for the fat soluble vitamins A, D and E.
On the other questions - baby rice, no idea. I usually feed DS in between the milk feeds, otherwise he wouldn't be hungry enough to eat anything or have the second half of the milk feed, but he is a bit older (8months) and was I think when I started weaning him