ryans
I would imagine most 5-htp is metabolised before it gets to your nervous system without the consumption of drug to block its metabolism. If any drugs are going to have an effect, it will be either the SSRI class you mention, or a COX inhibitor (most painkillers are COX inhibitors) as these increase serotonin in the brain by reducing its metabolism. Neurotransmitters are really complex though, its possible these drugs will make it worse rather than better.
The precursors to serotonin are metabolised though the kynurenine pathway, lowering the level of serotonin in our brains. This happens to protect us against infections, and occurs in response to stress, inflammation, infections, or things our bodies think are infections (like allergies for example). It might sound too simplistic, but your best way of increasing your serotonin is be as healthy as possible.
For me its quite clear, I have bowel disease, so my body thinks its suffering an infection even though it isn't. Through years of hard work I've found out what triggers it, and now each year both my bowel disease and screen intolerances get better.