OpenVPN on AWS: access to ec2 with their private IPs

[mag]

Hi there,
I have a OpneVPN instance in a subnet in AWS. I also have several other ec2 instances in that subnet. when I am connected to openvpn, theoritcally my remote machine is also in the same network. (my understanding: OpenVpn has a elastic IP and my machine IP also changes to the same one on the openvp). I am only able to access the other instances from my web browser with their public hostname. In OpenVPN setting, when I change Listen on all interfaces: Yes, then I have access to all the machines with their private IP. Here I have two questions:

1. IF: I change Listen on all interfaces: Yes. Does this make the network less secure?
2. What is the best way to access those EC2 with their private hostname?

Thanks