You definitely need more then just the SE. While IE for example has seen improvements with antiphising filtering it still won't block various sites that will dump things on your system. The same is also seen with other browsers since ways have been in use to get around those as well.
With Symantec grabbing Threatfire what would you expect since they have been known for bloatware over the years. That takes time to instill in a new program they acquire!
As far as running that along with the SE the only way to know is by trying both together while the SE was found to be unintrusive compared to the typical dilemna seen when trying to run two of the usual name av programs together. The Windows Defender is on all the time for most systems regardless of what other protections you have installed from 3rd party sources and doesn't see clashes being another MS additive.
(and like many previous MS bug chasers not very effective either!