I would recommend Reptomin (found in the reptile aisle) and earthworms only and would not feed the rest.
As far as salt baths, the 20L is fine for them. Use clean water, dechlorinate it, for every gallon of water in the bath tank use 1/2 tsp of epsom salt. Each frog needs to sit for up to an hour and each frog needs it's own bath water - don't reuse between frogs.
Repeat once daily.
I am a little concerned as it has been 2 and 1/2 months since you originally posted regarding their bloat. Can you post some updated pictures/videos of their condition?