Adam, well done for rehoming the tetras without a fight once you realized you didn't have the proper space for them. I myself have made that mistake several times when I was silly enough to listen to people at the petstores. I had a guy who INSISTED he was an aquarium expert sell me 2 male dwarf gouramis for my 10 gallon tank, and told me they were very peaceful and easy. It took me about an hour to realize these 2 were going to attack each other constantly until I took one out - 10 gallons isn't really big enough for 1 male DG, let alone 2, AND they are semi-aggressive - even after I removed 1 the other one almost killed my platies. I also promptly rehomed them both on facebook and learned my lesson. Now I research before I go to the petstore (BTW, if anyone is looking to expand from bettas a housing site of this forum, tropicalfishkeeping.com, is an EXCELLENT forum with active and helpful experts).
Also - fun story - the petco near me once tried to sell me 2 female bettas to be housed together in a 1/2 gallon bowl. They had a "factbook" next to the tiny plastic cups with the bettas that said that 2 females are fine to live together and that 1/2 gallon is the perfect size for this. I wanted to throw their book out the window and bring them a copy on how sororities actually work and the requirements for them. Unbelievable.