I would use flake for the fish but crush it almost to powder so it's small enough for them to eat. They won't need much as they are so tiny.
My frozen multipack has daphnia, bloodworm, brine shrimp and mysis. All of these are a bit on the large side for my sundadanio, which are bigger than brigittae. If you can find it, baby brine shrimp would be suitable.
I feed my 50 litre tank with a mixture of shrimp cuisine, New Life Spectrum pellets and New Life Spectrum flakes, just a tiny amount of each crushed/ground into small particles. Of course all three animals eat some of all three foods. And for 'live' food, I feed frozen cyclops.
They also get fed mashed peas when I remember.