I don't think anyone told you that a 3.5 gallon was too small for a betta. They simply said that it was too small for a betta AND tankmates. Many betta-compatible fish/amphibians like otos, african dwarf frogs, cories, and some livebearers are social creatures that need to be kept in groups, and a group won't fit in a tank that small. You also need to consider that many small fish, although small in size, have active swimming habits and won't be happy in a tank that small. Then there is bioload to consider. Also, there is more risk for aggression in a small tank since fish can't easily get away from one another. Overall, a 3.5 gallon is fine for a single betta but other than a single snail, I can't recommend any tankmates.
Regarding bettas in soda bottles and other tiny containers, my only comment is that surviving is much different than thriving....
20 gallon long: 3 adult Neolamprologus similis + about 11 fry of various ages; low light planted tank
20 gallon long:2 freshwater dwarf puffers (Puff Puff and Poofer); medium-light planted tank
10 gallon: 1 male betta named Wormy; low light planted tank
10 gallon: 1 male betta named Dante; low light planted tank
2, 5.5 gallon tanks that are currently empty (I see more fish on the horizon