Is there any way to specify priority for replication. or any parameter which guarantees something about replication (speed at which it replicates, number of minimum replicas to write).
Not yet. Not in core PostgreSQL streaming replication, nor in BDR or pglogical etc.
Right now you have "synchronous" or "not synchronous" and at most one synchronous node.
Does BDR has a configuration for differentiated services in replication.
No. It's mesh multimaster and all replicated data is treated equally. There's no concept of replication priority, nor am I sure how we could implement such a thing. Data is either replicated or not replicated.