If I'm reading your question right, go into the NAT rules and enable advanced outbound NAT. At that point, use the internal range of IP addresses for the 15 users (for example 192.168.0.16/28) and NAT them to one of your static addresses (using Virtual IP's set up in firewall menu), and then put a rule at the end for "catchall" using the final IP.
For example, if your network was 192.168.0.0/24 and you had public range x.x.x.1-x.x.x.5
x.x.x.1 is the WAN address of your firewall
x.x.x.2-4 are the 1:1 NATed ip's to maybe 192.168.0.10, 11, and 12 to give 3 users Static IPs (if using DHCP, make sure to map those MACs to 10,11,12)
x.x.x.5 would have an advanced outbound NAT set up as 192.168.0.16/28 for source (use DHCP leases to give to those clients in that range from 17-30)
Make your last NAT rule the one that NATs 192.168.0.0/24 to "WAN Interface IP" and you should be fine.
Let me know if you need more specifics.