So I'm currently doing up my dads old tank from 20+ years ago and I was wondering what I'm going to put in it, its a 20 gallon hex and will be fully planted when finished, I really like the endlers and cardinal tetras and was also considering putting a Betta in as a bit of a centrepiece as I feel like a tank full of tiny fish would look like it was missing something but I wasn't sure if that would work, I've seen a few things about Bettas being in communities online but was wondering if anyone had any input?
We usually advise against housing a betta with other fish because they are so unpredictable. Some do manage to live peacefully in a community, but most will object to having tankmates so it's best not to risk it unless you have a spare tank to move him into if the need arises.
When they are kept with other fish, tankmates need some thought and it's best to avoid very colourful fish or any with flowing fins/tails which he will see as competition.
You mention liking endlers and cardinals, but unfortunately, those can't be kept together as they have different water requirements. Endlers need moderately hard water and cardinals need it soft and acidic.
If you can post your tap water parameters for pH and hardness (available in your water quality report fro your supplier's website if you haven't yet go tets kits), we can offer some suggestions.