I want to use UCS users to authenticate in pfsense squid is possible?
The squid integration in pfSense has support both LDAP and RADIUS for authentication, both are services that UCS provides. So technically there is an option.
However depending on how you exactly plan to use squid your question might not be easily answered by a plain ‘yes’ or ‘no’. (Depending on explicit or transparent mode or when you want SSO etc.)
You might have to further think on how you want to configure squid for your use case.