Yes. You can do that. I have many Ethernet devices plugged into both sides of my MoCA bridges and they can all communicate with each other as if they were plugged into the same switch.
Regarding security, if the units do not support encryption, you can physically make your network secure by placing a filter that blocks out frequencies above 1GHz at the cable drop. You just need to make sure that your MoCA adapters are communicating at frequencies above 1GHz to be secure. Here’s an example of a filter that is designed for this.
