Do replication sets have to be identical when they contain sharded collections

Just curious, if you shard a collection do the replication sets in each shard have to be indentical in terms of configuration - ie number of hosts? eg If I had a replSet in shard 1 with 3 hosts and a replset in shard 2 with 2 hosts would this be ok?
Following that, if it is ok and I issued a query with writeConcern: 3 would this return succesfullly for documents that were stored on shard 1 and fail for dicuments stored on shard 2 as not enough hosts are up?

Many thanks,

Tony