Hi and welcome to the forum
How long has the tank been set up for?
How often do you do water changes and how much do you change?
Do you gravel clean or suck the gunk off the top of the substrate when you do a water change?
What sort of filter is on the tank?
How often and how do you clean the filter?
I don't normally recommend keeping Bettas with other fish, especially if the other fish are brightly coloured or live near the surface. The male Bettas often consider these fish as intruders into their territory and attack them. Dull coloured fish and fish that live near the bottom are less likely to be attacked.
Some Bettas are fine with other fish and some snap suddenly. There's no way to tell which fish will get along with others or if/ when it will snap.
I wouldn't add anymore bottom dwelling fish because there are already things living there. If the tank is established and gets regular water changes, and the Betta has been with something like Harlequin rasboras and was fine with them, that would probably be ok but you will have to monitor him for any signs of aggression.