Yes, but it's not as if you were passing a law or something. Your protocol is mainly about setting a common standard for syntax and semantics of the negotiation of file transfers among p2p clients, not about setting rules for the clients' interface.

In your proposal it is not a breach of the protocol not to support partial file sharing ( if it was you'd have to form a completely new network without all the older clients ), so it's up to the clients if they want to support it by default, optionally or not at all.