I would not keep frogs with a betta (or any other fish come to that). I did once and never again.
Frogs are almost blind, they find their food by smell. By the time they located it, the betta had eaten the lot. I tried all sorts of ways to keep the betta away from the frogs' food, but he always found a way. And being almost blind, if they see movement, frogs assume it is food. I found my betta with a frog clamped to his tail - he was swimming round frantically trying to dislodge it. I had to set up my quarantine tank (splitting the media) and move the frogs.
Shrimps can be hit and miss. My last two bettas lived quite happily with cherry shrimps; my current one ate them so I moved the survivors in to my main tank. With my last but one betta, I had an ever increasing shrimp population so he obviously ignored baby shrimps as well as adults; the last betta ate babies even though he left adults alone.
25 litres is fine for a betta and maybe a nerite snail. Or a couple of African dwarf frogs. Or a shrimp colony.
If you decide on frogs, ignore sites that say they live on nothing but bloodworm. As with fish, these should only be a treat. There are commercial frog foods which are better as the main diet. I used Zoo Med Aquatic Frog & Tadpole food.
