amphipods are too large for newborns. After a month or 2 they can eat them, but mysis are really the only reasonable option. IMO the BEST source is
http://www.reefnutrition.com/specialty_feeds.html#mysid they are lab grown and always arrive healthy.
I too was unhappy with Sach's mysis but part of the problem is his are wild caught. He is a great source for shore shrimp which you are going to need as they get bigger.
Once they start eating about a week after hatching usually 1-2 mysis per day is good, then 3-4 and once they make it to about 5, amphipods are okay to feed and then shore shrimp.