Can PostgresSQL cluster have synchronus and asyncronus standbys at the same time for the same dbs?
Yes,

There is a setting for this which is called synchronous_standby_names.

You can give expression like ANY 3 (r1, r2, r3, r4) which says "proceed commit as soon as at least any three standbys reply"

If you want to specify exact replication name just name them like r1,r4.

Here is sample usage;

ALTER SYSTEM SET synchronous_standby_names TO 'ANY 3 (r1, r2, r3, r4)';
SELECT pg_reload_conf();


Note: The standby names (which are r1,r2..) are defined by application_name option of standby's primary_conninfo setting value.