I'm really becoming disheartened with this forum - it seems that 90% of the replies to anyone asking for advice are just lectures about how everything people are doing is being done wrong, and I'm wondering how many of them are just regurgitating facts that they've read on the internet - and how many of them have actual real time experience with the animals in question.
Either way - being a 20+ year hobbyist, I get really irked when I see anal retentive people going entirely overboard on the care requirements of an animal. Mata-mata's do fine with both pleco's and deeper watered tanks, so long as you have somthing free floating for them to hang onto when they see fit, and provided that the tank has hiding places for the pleco - as well as ample space for them to not constantly be in the same area.
As far as tank mates go - I would be inclined to recommend a large shoal of tetra, but since you're going to have pbass it's probably a bad idea. Most predator's get confused by a large group of tetra's and end up not eating any, but I think pbass being apex predators would have no problems getting a mouthful.
Have you considered doing some redhook silver dollars? Maybe a shoal of discus? There really are not a lot of options in a 135 gallon for South American fish that won't outgrow it but still coincide with a matamata and peacock bass. Especially if you're trying to stick to a South American biotype.
If I misinterpreted what you wrote and you won't be keeping the peacock bass in the new tank then you've got tons of options to go with.
