I agree with the times mentioned by neon and choc chip in reference to the 4 times a day feeding although we sometimes gave Willow her first meal a bit earlier than that as she was and still is a greedy sod
Willow came to us from her breeder on Beta puppy, she would poop for England at a guess about 10 times a day it was very runny. In the end we had to give her chicken and brown rice to firm her up. Any new food should be introduced gradually (over about 2 weeks mixed at first in small quantities with the Beta puppy food) However as Willows poops were runny we ended up keeping her on the chicken and rice and introducing a new food gradually from there.
From memory Willow had about 25 gram of food at each feed initially. We got a free bag of food with a measuring cup when she came home to us.
I hope this helps. If unsure what kibble to give there are plenty of threads on here about the good quality kibble. Willow is now on Barking Heads and loves it, she is in great condition and it contains good ingredients. She only poops about 3 times a day now too
In terms of feeding on demand ive not heard of that, if we fed Willow on demand she would of popped for sure
In reference to the supplements ive not heard of that either but im sure someone will be along soon to offer some advise on that for sure. i know we didnt give Willow anything in addition to her food at that age