I am not sure what you are planning to do, e.g. What kind of applications that you have on your Droplet for your devices? What kind of services that your devices need on your Droplet? Are you sure that you need VPN at all for that? 🙂
Well... If you are not comfortable with the "LAN based stuff", e.g. subnet and routing, then we can worry about that later. But at least you need to know some basic stuff, like making sure that your applications listen to the VPN IP address of your OpenVPN server as otherwise your devices can only connect to it but doing nothing.
As you already have your OpenVPN server running and you have 1 device successfully connected to it, I would imagine you just need to:
1. Create certificates and keys for your other devices
2. Copy .ovpn file that you already have, for your other devices
3. Change the certificate and key on the copy of the .ovpn file for respective devices, with the ones you created on step 1.
I still think it would be better for you to understand more about this by reading the how-to page. At least on the part that is relevant to what I explained above on
this page.