Splitting one class between propulsion systems weakens commonality

Could half a class use nuclear-gas propulsion while the other half used gas turbines and diesels?

Not sensibly; two propulsion architectures inside one class would create avoidable maintenance and training burdens.

A mixed class would give nominally similar ships fundamentally different engineering plants. The split would burden maintenance and training. Common hulls would no longer provide common support requirements or a single engineering skill base. Fleet standardization matters beyond outward ship shape and weapons fit.