gouramis are alot more sensitive than bettas. when i was really young, i had one in an uncycled tank (wonderful petsmart education!) and it was what I know now sucumbing to ammonia poisoning. I tried to keep him in a seperate bowl, but he died shortly afterward. So while they can survive in low oxygen levels, they cannot tolerate high toxicity for very long, just like any other fish.
The little containers retailers use, are ok for short term but not long term. And most I know of dont change the water everyday, they put some blue chemical in the water and change the water like 1-2 times a week. I'm sure they have a pretty high mortality rate, and thats easy to see if theyre not quick on removing the dead ones from the shelves, like the walmart by me.
I believe bettas like smaller spaces, like 1-5 gallons. They like to have their own territory that they can patrol. Some Ive seen in big tanks get freaked out, and then having to swim so far to the top. I know some people have had luck with them in bigger tanks, I'm just posting my observations. I also have noticed they prefer little to no surface agitation, which in a community or heavily filtered tank is hard to come by.