Clams are hardy in that they will manage to survive for quite a while in poor (for them) conditions, but eventually it catches up with them and they die. I have honestly never heard of someone succeeding to keep them alive and healthy in an aquarium with fish. They need different things, and are usually used as short term cleaning. They will clear your tank of green water very efficiently... But they requirements for living anything longer than a few months are pretty hard to manage. I believe at least six inches of substrate, high levels of calcium, a constant current with a flow of food, etc. I also believe that the common chemicals used to treat water are harmful to them, so you need to start with a base of RO water...
Seriously, they are a huge pain to keep living. I would give up on keeping them with a betta as they absolutely require a good current of water to stay alive for any decent amount of time.