For my guppy fry, I feed them Hikari First Bites powder that sinks to the bottom. Your multi fry seem to be a bit bigger than guppy fry. I also have a few mystery snails and shrimp to eat the leftover food.
I don't know if the same goes with multi's, but I would definitely try Hikari First...
Petco sells Blue Velvet Shrimps, and I have purchased many from them.
Cherry Shrimp are one of the hardiest shrimps to keep. They are very vibrant and should be pretty easy to keep. When I had mine, I keep them in a 20 gallon tank with guppies. I fed them bottom feeder pellets, and most of mine...
Yes, shrimp eat bio-film from the top of the tank. I would start feeding them boiled green vegetables.
In my opinion, that tank is still lightly stocked, with shrimp taking up the minimum space in tanks.
I don't know if I would consider this a culture, but I've seen many Petco's around my neighborhood that sell canisters of flightless fruit flies.
Depending on where you live you can find a Petco anywhere, at least where I live, but I understand if Petco isn't the most convenient place for you.