I've been on 90-120mg DXM extended release daily for about 1.5 years for therapeutic NMDA antagonism, and due to the Serotonin Syndrome scare, it took me a very long time to taper from 0 to 2.5 to 5mg to 7.5mg to 10mg Prozac once daily, which I've been on for about 5 months. A lot of doctors will say that a dose as low as 10mg of Prozac is almost a placebo as an anxiolytic, and they're probably right, but combined with the DXM, it has very powerful 5-HT (serotonin) reuptake inhibition synergy.
Here's the problem: Prozac is one of the longest acting pharmaceuticals out there, which is why you will feel no withdrawal if you miss less than 4-5 days of dosing, as the half-life for fluoxetine and norfluoxetine are ridiculously long, so don't think you can skip a few days of Prozac then trip of DXM. It will literally take 3 weeks without Prozac for the active metabolites to be significantly reduced to low enough levels where the risk of SS when mixing with DXM becomes a non-issue.
If you've been on Prozac for more than a couple of weeks, I wouldn't recommend it at all. If you're dead-set on tripping off an NMDA antagonist, try some Methoxetamine or pure (vial) Ketamine instead, as they have no appreciable 5-HT activity.