I have had daphnia always in my fry tanks. Live plants seem to be key
as for the water mites, they do feed on the weak fry. Making sure your fry have sufficient food (as mentioned egg yolk is good for the mites not to eat the fry, and fry get nutrients). I've only had some water mites but at that stage my fry were big enough to eat anything in there.
I find the easiest stuff to cultivate grows on their own. Infusoria, daphnia, are the ones I stopped worrying about. Always have another food source! Whether that is vinegar worms, white worms, bloodworms or microworms... And BBS is not that hard either :D trust me. My four week old fry are devouring thousands of BBS within minutes. You can never have too much food for them!