I have two ADFs in a ten gallon with a snail and Thaddeus, my plakat betta, they do fine. I have a sand/gravel substrate and feed them frozen bloodworms as treats with a staple of HBH Frog and Tadpole bites. They love it. :3
When i had just one i kept her in a 2.5 with a betta and nothing bad happened, but i did my water changes often and kept the water and tank obsessively clean.
Each full grown ADF needs 2.5 gallons to itself, so you could have two in a five gallon, but you would only be able to have the frogs in there.
A SINGLE FROG WILL BE FINE IN THE FIVE GALLON.
The only reason we say they are social is because they are more entertaining to watch when there are more than one. Thats all.