Hi
With a plec and a stripped catfish you already have 2 south american fish in there so I would potentially look at doing a tank around that theme.
What kind of plec is it that you have? If it is a common there is a chance it will outgrow your tank but a 75g is a better home than most they are sold to. So it could work out for a while. If its a Gibbicep or a Sailfin your probably looking at either a bigger tank in the region of 180 gallons or rehoming them for a smaller species.
The great thing about Amazon themed tanks is the variety that comes from one region.
2 Blue Acaras - or any kind of South American cichlid (within reason, I wouldnt do an Oscar or a Green or Gold Severum in here but most others should be ok)
5 Marbled Headstanders - larger schooling fish, interesting pattern and behaviour
12 Disk Tetra - any kind but your just after that round shape, I think they will do better than the smaller torpedo shaped tetras like Neons. But you have a choice here of things like Black Widow Tetras, Flame Tetras, Yellow Phantom Tetras etc
5 Porthole or Flagtail Catfish - for a bit of action along the bottom but there are other options here
1 Plec (depending on species)
1 Stripped Catfish
Tons of other options though just depends on what you like really
Wills