Damn. I gently offered my mother the chance to go buying our new betta to the fish store for the 10 gal tank. She got mesmerized by the community tanks so she wasn't impressed by the few available bettas and all of a sudden she wanted other fish for the community tank because "a betta was too little of a fish for it". After some research, I came to the conclusion that Black Phantom Tetras were small and peaceful enough and bought 4 of those who are in quarantine now.
Their lack of fancy fins leds me to believe that a betta wouldn't fight with them. Do you have any experience regarding the combination? I'm having a 5 gal tank as backup just in case.