Simulation-based security provides more natural and more powerful definition of security than game-based security. I have seen the simulation based approaches use the game-based proofs in-part to prove security of some parts of the protocol. For example, to assess the security of a protocol for round complexity or total messages exchanged during execution of the protocol, a game-baed approach is taken, but the security of the protocol itself regarding the framework (in my study the UC framework) is proven by Ideal/Real paradigm (i.e., simulation based approach).
Question: Under what circumstances can we use a game-based approach to prove security of a part of the protocol or the whole protocol when the protocol design must conform to a simulation approach? Can we use this approach at any point for any reason as long as it is not relevant to security of the whole protocol regarding the simulation-based framework?
Let me explain it through an example: I am studying a group key exchange protocol over UC framework (simulation-based), but the protocol uses encryption schemes which are CCA-Secure (proven by a game between adversary and some oracles) or CCA2-secure (proven by a game-based approach too), signatures must be existentially unforgeable (game-based approach too) and finally the communication cost of the protocol is calculated and analysis of the protocol security is done through 10 or 11 games between adversary and simulator. Finally proofs are made to show that the protocol conforms to UC regulations.