I have added newborn fry in with my 1 wk, 2 wk, 3 wk old fry and been just fine... HOWEVER the other day I added newborn fry in with my 6 wk fry who are already 1" and they treated them like a special feast--- great big fishie "pack attack". They swarmed and the couple fry I added in there were almost instantly killed and the 6 wk olds were swimming around with the babies sideways in their mouths. I was absolutely shocked!! So, it looks like 1-4 wk old fry are good with newborns...
Big mistake I"ll never make again.