They are. Brine shrimp are often referred to as "Popcorn". To us, Popcorn is Great. But has little to no nutritional value. Brine shrimp is pretty much the same. Vitamin enriched kind of helps with that. Some are even gut loaded and good for quickly filling the fish. Brine are a really good way to get a picky eater to get use to food being served the way you serve it. Eventually, They just start going after the food right off the bat thinking its the yummy brine. That will help adjust them to eating brine. My Black ice only ate pellets when I got her almost a year ago. Now, she eats anything that hits the water. LOL
As for how much to feed. I thaw my little cube in RO Fresh water. Than I poor out the "Waste" Water into a Fine Mess net. I than place the food of choice back into the container and fill it with some tank water. Often times the water the cubes are frozen in have nitrates & Phosphates in them that can lead to some serious algae issues. Especially in such a small tank. I only feed a little bit. Enough so that each fish gets a nice full belly. Than, the rest goes into my fridge until my next feeding.