I'm by no means an expert, but regarding porridge... I don't see why not. My DS has been having porridge from about the same age with no issue. He tends to get blocked up too so I give about 2/3 oats to 1/3 stewed/finely grated pear. I probably only give him 1/4 of a banana every week or two as this is a major culprit for constipation - poor little love, he LOVES bananas and would have them every meal if I let him! Really pushing the fluids and avoiding too much starchy foods and meat has generally helped. And he eats enough fruit to feed a small nation!

As far as I know, pooing once a day is fine as long as they're fairly soft and not causing too much pain to get out. My DS still makes enough noise doing a poo to silence a crowded play centre with his grunting!! But he gets 'em out and seems happy enough, so it's all good!