Male Betta Tank Mates 40L

Sorry for slow reply.

A male betta is definitely best kept on its own.  If you're really keen for him to have tankmates, the choice is very limited / well-nigh impossible in a tank of that size (most species require kept in groups and in a tank of minimum footprint 60x30cm), and you'd need to be prepared to have a back-up plan of a separate tank for those fish if the betta ever takes a sudden dislike to his tankmates.  Would a larger tank be a possibility for you?

What is your water hardness level in CaCO3 or German degrees hardness (ie actual numeric readings/measurements)?  Your water supplier website ought to have that information contained on it.  That way, we might be able to identify a suitable species compatible with your water and potentially compatible with the betta.

In that case (ie larger tank not possible and inability of spare tank to transfer fish into), you're indeed best sticking to the single male betta on his own.
Up-to-date info on care for bettas is at: https://www.seriouslyfish.com/species/betta-splendens/
 
Your water is nice and soft, so plenty of fish to choose from for your other tanks if you decide to go down the tropical line in the future for those.

It depends on the betta. I have kept several bettas one at a time over the years. One was particularly laid back so I tried some red cherry shrimps with him and he ignored them, including the baby shrimps. The shrimp numbers increased. After he died I got another betta, and this one ignored the adults but presumably ate the babies as the numbers stopped increasing. As the shrimps died of old age I replaced them with more from another tank I had. Fast forward to the next betta, and this one dismembered adult shrimp, there were shrimp bits all over the tank.

The problem is that you don't know what an individual betta will tolerate till you try, and that can be costly if you find out he won't accept any tank mates.