Oh for sure they can "live" in a .5-1G but who wants to put there betta through that??. The bigger the tank the more lively the betta becomes. Also if you think they are an easy fish to care for you are right but in bad water conditions they don't swim at all they just float there. But kept right they thrive.
Almost all albino frogs are going to be a clawed frog rather than a dwarf frog. The hardest part about keeping an ADF is ensuring that it eats properly. A tight lid is a must.
Here is what my adult (female I think) looks like.
As for bettas, a 2g is "do-able" but a 5g is comfortable.
How big do dwarf frogs get? Petsmart sold Regular ADF's for $2.50 but XL ADF (oxy moron anyone?) for $6. The regular's were the size of a quarter or so, but the extra larges were about 2-3 times that size. They looked like the same size, and I doubt they were the clawed species because they were kept in a tank with little fish.
ADF's stay small...at or under 1.5" mostly. I bet the big ones you saw were clawed frogs; when they are young, they aren't that harmful to fish yet- and are often kept in small-fish tanks.
I mean, look at the size of the glosso leaves in my pic compared to an adult frog. Pretty small little guys.