So here's what I ended up doing. Since the setup was basically the same as another setup I had done that was working, I figured I'd just mimic that and hope for the best. I don't have the failover configured yet, but here's what I have.
ISP Gateway (x.x.157.17/29) <-> pfSense WAN (CARP x.x.157.18/29) <-> pfSense LAN (CARP 10.205.154.66/21)
1:1 NAT
x.x.154.66/26 -> 10.x.154.66/26
x.x.210.0/23 -> 10.x.210.0/23
NO VIPs (outside of the CARP WAN and CARP LAN)
I guess since my ISP routes the two subnets (above) to x.x.157.18 as the "next hop", pfSense automatically handles those requests since they match the external IP address of the 1:1 NAT entries. This works out nicely, since I can't see any reason why the subnets wouldn't fail over to the secondary server because neither server is "advertising" that they control the subnets; they just utilize the 1:1 NAT to map them after they receive the packet.
Before I finalized this setup, I took a look at the "Other" and "IP Alias" VIPs, and I noticed those can only be done on individual IP (same thing for CARP). This obviously wouldn't have worked for my setup, because I have well over 500 IP addresses.