I don't know much about Threatfire except what I see at their site. They
seem to feel it is compatible with standard a/v scanners--but since every
vendor out there is looking at the same kinds of "behavioral" detection
issues, I do wonder whether multiple vendors will end up stepping on each
others toes over time. The reviews I saw cited were not recent, and were in
only one publication.
There is a single API which is shared by all real-time protection
applications on a given system. It is best to have only a single
application using that API. So--it's fine to have multiple apps which do
separate def updates and scanning of whatever kind--spyware, antivirus, etc.
Limit the real-time scanning to a single application though. Some apps can
be installed and have real-time protection turned off--so you can switch
between apps, but this is really not something the average user should
bother with--pick a vendor you trust and use them as your primary