1. Plecos hide. If you keep them and do not provide adequate cover, you will have problems sooner or later.
2. The Jack Dempsy is named after the famous boxer. That common name should be a clue as to it's temperament.
3. Most fish see smaller fish and eggs as food.
4. The normal dividing line between what any given fish might eat in term of other fish is, " Does the potential meal fit in it's mouth?"
The thing about keeping fresh water fish is that they come from isolated places, a river, a lake a pond a rice paddy etc. There is a wide diifference in parameters from location to location. The one common thread is the water is fresh (very little or no salt i.e. sodium chloride). This leads to the potential to mix species which would never meet in the wild. It is the responsibility of the fish keeper to make sure 110% that mixing species which would never meet otherwise might will be OK to keep together. If in doubt, do not do it.