Copilot Chat Adoption Feedback : Model Selection Governance and Prompt Library Reusability ?

Have you tried accessing the Prompt Lab (That's what the library/gallery is called in our version of Copilot anyway) through the three dot menu in the top right corner in help and tips?
This works in an existing conversation for me. Based in Australia, milage may vary depending on location. 
I don't know why Microsoft has hidden it there, took me a while to find it the first time. 

Locking or enforcing a specific model may be possible in your tenant by a system admin depending on setup, for example by enabling the desired one and disabling all others. 

It can be handled by a specific agent created and deployed by Copilot Studio too, you can choose the model for the agent, if one is created through that process. If you did lock the model on the admin side that would likely affect all users at all times though, so everyone would have to use only that model in your tenant. This may not be what you intend.