In my opinion it is possible the jewels could have killed the poor plecs.. They tend to attck the fins first n after awhile the plecs lose balance while swimmin n gets more fragile.. Plecs have hard armour but a vv soft stomach... So a good knock or bite on the stomach while it is swimmin helplessly is all it takes to kill it..
I have seen jewels kill fish similarly before. I had a solo male kill a pr. or labiatums(red devils),& a convict. in 36 hrs.not by choice I was away for a few days. the male labiatums was tore to shreds & humanely euthanized the other 2 were dead by the time I found them. In that small tank you are pushing it with that many jewels.
With that being said though-P's certainly dont have ne personality like cichlids do----I have mainly switched over to cichlids now.....Just for that reason....Plus pound for pound-they would woop a p.....