I have a betta in a 15 gallon with some guppies. My LFS got some gardneri killifish and I was planning on moving the guppies and getting the gardneri and putting it with the betta.
From what I gathered from the post It looks like he/she was going to move the guppies from the 15 gallon that are currently with the betta and put a gardneri in with the betta. I am not sure what else would be helpful. I do know some bettas don't get along with other fish.
Betta's are not community fish and shouldn't be treated as such, I'm afraid. They may appear to 'get along' with other fish in your tank, but it's always a ticking time bomb that we're not aware of. In the end, either the betta with kill all of the other fish in the tank or die itself from stress weakening it's immune system. Betta's are best living alone, despite what YouTubers may recommend