I've had a hard time keeping my ADF, Darwin, with other fish. I've tried keeping him in a 28 gallon community tank, where he got nearly no food at all. So I tried a 5 gallon hex with a betta, but it didn't go much better. The betta would snatch up anything I dropped in. Even if I was able to sneak some down to the frog, the betta would soon find it on the tank bottom and eat every last bit. I've had a couple times where I thought my betta was going to explode becuase the ate so much of the frog's food!
ADFs can learn to follow your finger to find the food, but it's not the most reliable thing in the world. They get distracted easily.
Another thing you could do is stick a tube (not see-though) down to the bottom of the tank in a back corner, and drop the food though it. That way the the fish won't have a chance to snatch it up before it reaches the bottom. But, once the fish do catch on, you'll have to find another way to do things.
My ADF now lives in a 5.5 gallon with a Dwarf puffer. They both eat live bloodworms, but the puffer won't eat itself to death like the other fish will. He takes a worm or two and leaves the rest for the frog. I still watch the ADF eat everyday though, just to make sure he got enough.