Can I use 1:1 NAT on my WAN IP
Yes, 1:1 NAT may be used from the WAN IP address to an internal IP address. But be aware that this maps every port and services on the firewall will no longer be reachable from the outside. To reach the firewall from the outside, port forward entries must be added to negate the 1:1 NAT for the specific ports on the firewall to be reached.
If there is only one WAN IP, and a Linksys (or other SOHO router) style "DMZ"1 configuration is being attempted, consider only forward the ranges of ports and protocols that are absolutely necessary, and use appropriate firewall rules to protect access to these ports.
1: That term is used very, very loosely. See the Wikipedia Definition of DMZ for more information.