Is an anomaly possible if a serializable transaction interfers with another isolation level?
PostgreSQL wiki https://wiki.postgresql.org/wiki/Serializable#PostgreSQL_Implementation states:


  Any transaction which is run at a transaction isolation level other than SERIALIZABLE will not be affected by SSI. If you want to enforce business rules through SSI, all transactions should be run at the SERIALIZABLE transaction isolation level, and that should probably be set as the default.


In your example since transactions A and B are not both SERIALIZABLE, anomalies can happen.