If the tank is filtered, if the tank is cycled, if you have a cave and plants so the frog can hide, if you can feed frozen bloodworms (not freeze-dried), if you can do 30% water changes twice a week and if you have a backup plan, then, yes, you could have an ADF.
Make sure the lid is a tight one as ADF can escape and die rather quickly. I wouldn't put two ADFs and a Betta in a 2.5. Can you get at least a five gallon and keep the 2.5 as a backup if it doesn't work out? Then you could have two or three and they do like to be in numbers as they're very social critters.