@Itwerx:
Depending on your hardware and network configuration it's possible that there might be a more robust solution, but if it's just a single pfsense box with the multi-WAN being the only difference from a typical configuration, then yes, that should be fine.
You're right i've a single pfsense box for 2 wan, 1 lan and 1 DMZ plus another ethernet card connected to another pfsense box which serves only as a proxy server. Why I decided to use a second machine just for the proxy? Simple, is an old PC and in this way i can solve the problem of squid in multi wan configuration. ;)