First, I would like to preface this with the fact that I have never kept angelfish, so what I recommend is based off my experience with fish fry in general.
For fry, especially for some sort of sustained food, I would recommend either vinegar worms or baby brine shrimp. These, since they are living, won't decay and ruin the water when not eaten immediately. Unfortunately, if you do not already have a colony of vinegar worms or BBS, the only method of getting these is from a fish store (and the one that I get mine at has point blank told me it's better to do your own, since you can feed them stuff to help your fish grow instead of whatever the shipper for the store does [if they do anything at all]).
When it comes to when to put them in the tank, I would as soon as you can. I would also start with as clean of water as you can, though you might need to drip acclimate them to the new water parameters. Babies are sold in stores both because everyone loves puppies but full grown dogs stay at the pound, as well as they are faster to adapt to changes (and lastly because who has time to wait grow all the fish out before selling them to make their money back). Old or young, one should always drip acclimate a fish to a new tank, as it is the shock of totally different parameters that kills, not the parameters themselves.
As far as what to transfer, every tank should have some sort of live plant (my opinion, both because they look nice and I'm lazy, but still merely an opinion) to help remove nitrates. If the fry that you have will eat plants, that will also help with feeding while you are gone. I put java moss in all my crayfish grow-out tanks, because I know it will get eaten and if they are eating the plant then they are not eating each other (crayfish are cannibals), as well as gives them places to hide. However, if Angelfish don't eat plants and you are willing to do all the necessary water changes to make sure the nitrates stay very low, and there are plenty of places for the fish to hide and feel safe, then the plants are not needed. I recommend some sort of plant for nitrate removal and hiding places, but that's all it is: my recommendation.
Hope this help