1) it is ok for baby not to poop everyday. Once in 2-3 days also ok, as long as baby poop not too hard and not constipated. Sometimes it's just not daily. U can consider adding a dash of water more to his milk
2) why don't u stick back to the one that worked for ur baby instead of re-finding something else? It could be a hit and miss. Hopefully u did not just suddenly make the switch caz babies will sometimes reject the taste - the transition has to be smooth. E.g. 4 scoop milk powder -- day 1 is 3 scoop A 1 scoop B; day 2 is 2 scoop A, 2 scoop B etc
3) base on baby age - u can see the health booklet. Some baby gain fast than other so it's hard to gauge too