Serotonin levels are lowered during REM sleep, but at an intermediary level during lucid dreaming. Therefore I would expect that taking SSRIs (enhancing serotinergic transmission) would reduce REM length and perhaps vividness. But it could also, like you said, increase the chances of lucid dreaming.

The neuromodulatory systems you are trying to fiddle around with are very complex though, so who knows what's really going on.