Haven't tried it, but I was thinking along the lines of otocinclus, and maybe a school of black neons, or some freshwater shrimp...
Maybe cories? Feed the cories some sinking pellets and feed the ADFs frog/tadpole sinking bits...
Some fish make it really hard to feed ADFs, so try and choose tankmates that won't steal your frogs' food, or bully them.
Maybe an enormous snail? I haven't tried any of these, so be careful... what works in theory may not work out in reality. It may be best to get the frogs their own 5g, plant it up, and maybe toss some shrimp in there for variety.