Because bettas are territorial fish, we do not want to mix them with territorial fish in a small tank - the worst it would do is encourage the fish to fight over it, the best would be heightened tension and stress. In a 60l tank, the betta IS the centrepiece. The tank is his domain.
Take care stocking your tank further, especially if you're a newer fish keeper. Overstocking can more easily lead to a decline in conditions down the road, which would affect the livestock in the long run. In a tank like this can potentially go several years with no losses, but many new fish keepers find them dropping off much sooner, I know they did for me when I didn't know what I was doing. Right now your tank sounds lovely, a big school of those little tetras must be stunning. Something I would add to the setup as it is right now is shrimp. a bunch of ghost or cherry shrimp, or a few Amano shrimp (but just one type) - this adds an interesting aspect your aquarium doesn't have yet but adds efficient scavengers that add minimal bio-load to the tank.
I had a betta setup for a number of years in a 15 gallon, which I believe is about 60l. It was live planted and the stocking was 8 cardinal tetras, 1 betta, 6 Corydoras stein., a few Amano shrimp, and trumpet snails. It was an interesting mix of creatures without any overstocking. I'm just starting a similar one now, but this time with 150 litres and apistogramma (or maybe Rams or Keyholes, I haven't decided) instead of a betta. And now that you mention it I think a huge school of embers might be pretty cool.
