Is it possible to create a private network using ganache-cli?
Can a ganache-cli running on port 8545 connect to ganache-cli running on port 9545?
Can we use ganache-cli as a node? Or it is a complete network?
This is not possible. It's not the purpose ganache-cli was designed for.
However, you could fork from an existing network with the -f parameter. See How to fork Ethereum from the current state?