Confirm please: a 5" syno ocellifer ate whole a 3" pleco? I struggle to imagine this is possible. Synos mouths are small and they may kill a tank mate out of territorial aggression but not eat them, not swallow whole for sure.
What you are describing with the syno and its victim plecos I think is possible albeit never happened to me. Something needs to change - a larger tank, more hiding spots, dither tank mates, etc. It is not normal by any means for a syno to be killing its pleco tank mates.