I also want to ask the same questions. But you've asked first, thanks!
My bb is also very thin (now he is 9 months old) Yesterday we brought him to see doctor because of sick, the doctor said that we should let him eat more milk instead of congee if we want him to gain more weight.
But I think we still keep on feeding bb with congee, because it offers different nutrition to bb, if bb eats less congee, then we will give him milk immediately at that meal.