I have kept male bettas with the tiny celestial pearl
danios, and I don't recommend keeping the two together due to temperature incompatibilities, but I had no choice at the time while we waiting to rehome them.
There's nothing wrong with tank mates as long as you find suitable ones that meet betta temperature preferences, and that they aren't speedy and nippy fish, which can annoy and harm the betta.
If you do any tankmates, then I recommend
ember tetras, chilI rasboras, glowlight tetras, or possible harlequins, but they aren't one of my
prime picks.
However, you can keep bottom dwelling fish like cories or some plecos with bettas, as they are very unlikely to do anything to harm the betta, it would be the other way around in this case.
I have kept one of my old bettas with cories for a very short time, and I also know that my brother had his betta in a community tank with cories for many months, and it was perfectly fine.
Neon tetras are not temperature compatible with bettas, and I have said this many times before in the past, they require a bit cooler of water than bettas prefer, so they shouldn't be kept with bettas.