On an existing LAN with DHCP and gateway,
I have a short list of Static Local IP addresses that connect with each other only.
They are to be isolated from all other traffic.
They are to be blind from all pings, polls, sniffs, etc., from any address not on the list.
I prefer not to create VLANs on switches that serve these IP addresses.

Instead, a "Firewall" router/switch will be devoted to each of the short-listed static networked devices.
I have a hEX Lite RB750r2, and will connect a device to one port, and another port to a LAN switch.
What settings will quiet the non-list activity at each device?

The WAN port will only be used for local configuration.
Do I need to bother changing the RB750r2 IP, subnet, gateway, whatever?

Bridge the two interfaces and turn on the IP Firewall. Create your rules.