Im not sure that you can tell rslinx to use a certain port for a certain device. I think with rslinx the default ports used for comms are fixed. So if you had multiple plcs you couldnt use port forwarding unless you had them change the port forwarding rules for each plc individually.
You can't, RSLinx uses port 44818 and it cannot be changed. AB tech support confirmed this, otherwise port forwarding would have been the easy way to go with each PLC using its own port.
VPN does provide security, but my needs are only to connect to a PLC to do some troubleshooting or reprogramming, so I would be connected for a short period of time. If I had some scada polling the PLC continuously, then security would be an issue.
So I'm still not sure if that "Remote Devices via RSLinx Gateway" driver requires RSLinx Gateway on the customers network. AB tech note 66666 says nothing about needing anything on the customer's end. That tech note closely describes my setup (but with Micrologix, not 5/05s).