One question I have for you, I have read all your previous posts, and I'm just wondering what good can come from these ideas of combos you have? A 55g is like bare minimum for one oscar, oscar and devil= badness to come, oscar + piranha = death for oscar, 2 pairs of convicts will not happen when they begin breeding!! Convicts have been known to kill oscars with relative ease and I'm sure a young red devil would not fare too well either. IMO a step back is needed and you need to decide which direction you want to go in... keep the one oscar and some small dithers/bottom feeders, the red devil with some others, or a pair of cons etc, or one piranha. GL with whatever you decide, but you need to remember all these fish have different personalities and aggression levels, the oscar is fairly mellow, convicts are always edgy and when breeding are crazy, and red devils in general have high aggression levels all the time.
Is the 55 a grow out or your only tank option?