Labrynth fish simply means that they have an organ that allows them to breathe atmosperically, rather than through gills underwater. That is why they go to the surface to breathe. Bettas can be kept in smaller containers because they are not big swimmers - they would rather lounge out then expend energy swimming. Dwarf gouramis are much more active than bettas (at least mine are
) and a two gallon is far too small, in my opinion. I suppose you could keep one in a 5g, but a 10g would be better
.
So, how they breathe has nothing to do with what size tank they need