Like I said before there is no need at this age to wait. If he isn't interested in 2 or 3 meals give him 1, but if he is interested there is no need to hold him back. And there are no rules on cereal or veggies or how to mix them. Mine did prefer each meal to be quite focused on one or two items rather than a plate full of different things but there is no harm at all in offering a variety.
The hour wait between milk and solids helps to
1. let him get a bit hungry for the solids
2. continue his association that milk is a different meal and to be taken in full.
If solids come directly after milk one of two things may (but not always) happen, that he doesn't have room in his tummy for the solids so doesn't eat as much as he might have wanted, making him hungry earlier than otherwise, or that he holds out for the solids knowing it is coming right after the milk so in effect drops his milk too early too rapidly in preference for solids.
Solids adds to milk feeds in that his overall calorie intake will increase with age but rather than increase just milk he gets milk + solids. He can only do this if there is space in his tummy for the solids. In the early days it can feel like you spend all day feeding but that passes quite soon as you get into the swing of things and as he become as more proficient eater. I can remember a solids meal taking a good 45 mins in the early days but that fine as it is their activity, they learn so much playing with the food and exploring it qualities. After a while though he was finished a meal in 10 mins eating far more but more quickly too.
WRT constipation or diarrhoea - depending on your LO there are a huge range of foods which can do either. There's a great thread I found really useful and after reading it I was able to control my LOs bowel movements naturally through food balance. If he became a little too firm I gave a prune or two, if he because a little too loose I gave him some carrots. It was amazingly simple so long as I didn't let it go too far one way or the other. I see this post looks quite new to me, the link at the bottom is the thread I read which I found helpful
Constipation and Laxative Foodsposted same time as jessmum