With IP Aliases you can assign each IP you got from your ISP to your WAN interface. However, this isn't necessary for your goal, since you have assigned the hole net segment (/28) to WAN if, but it's an advantage in clarity for handling the IPs in pfSense, I think. And it's recommended.
With 1:1 NAT and port forwarding you can handle incoming traffic (into pfSense), but you want to impact outbound traffic here. So you will need to configure outbound NAT for your requirements.
On the outbound tab in firewall > NAT select "Manual Outbound NAT rule generation" and click save. Then you should see a list of automatic generated rules for all your assigned subnets under mappings. Edit these rules or generate it manually if they don't exist, under source, choose the subnet you want to handle, leave protocol, source port and destination to any and at Translation address you can select the IP Alias you have defined before, if you don't select Other Subnet and enter IP and mask below. Leave the translation port to any.