Consensus comes with a lot of overhead if you want to achieve Byzantine Fault Tolerance. The number of messages per canister can vary a lot between different canisters and over time. Therefore we decided to batch messages for sets of canisters hosted on the same subnet and let them share the overhead.
You are right, different canisters have different requirements wrt replication, storage etc, …
At the moment, developers cannot choose between different subnet types, however we are working on bringing more types of subnets to the IC, as outlined here A Technical Overview of the Internet Computer | DFINITY | The Internet Computer Review. It will not happen over night though, there are technical hurdles to overcome.