All of the pikes I brought back from Uruguay - celos, missionaira, minuano - were eating bloodworms on day 1...and prepared foods (Kensfish flakes, NLS, etc.) within days. Mine also went crazy on torn up superworms.
From which river are the missionaira? Mine lurk and pounce on food!
I used buenos aires tetras as dithers...the bigger ones made it...the smaller ones were eaten. After quarantine, I dropped in a bunch of Archocentrus centrarchus grow-outs at about 3-4" and they all did great.
I kept 8 missionaira and minuano together in a 40g breeder. I think that they key is to have lots of PVC litter and other hiding spots....and feed well. The missionaira will hide in tubes not much bigger than they are (3/4" tube for a 5" fish).