For the fire-bellied toad, you can try to fatten him up a little bit with the occasional waxworm. The rule of keeping the prey to the size of the space between your frog's eyes does not apply with soft bodied worms like these. If you do decide to try the waxies, I wouldn't give him more than 1 or 2 a week, don't want him to become addicted or obese. Silkworms are also another option.
Silkworm Farms sells trial size silkie kits that come with food for the worms too. I'd stick with the juvenile size if you go that route, and these can also be fed as a staple if your frog accepts them. Sometimes they can be quite picky, so I'm not sure which way will work best for you, guess it'll just have to be a trial and error sort of thing.
As for that ACF, maybe try frozen bloodworms or freeze dried brine shrimp to entice that one to eat. I never had problems with any of mine being underweight or picky eaters. They loved the brine shrimp. Good luck with both, hope this helps you out.
