Hey, i'm going shortly to my lfs - I wish to by myslef a golden nugget if my water quality is perfect however i have 3 other common plec's. Size is not the issue here but you say they get quite aggressive..
Tank size would definitely be an issue especially if all the fish in your signature (which doesn't meet forum guidelines. See here) are in that tank. There are some pretty big waste producers. As for the plecos, I've never heard of them being aggresive at all so I don't know where that came from. Have they got them confused with chinese algae eaters which do get very aggresive as they get older?
Well I heard on here that there are some plecos that won't tolerate other plecos and will be aggressive during feeding or when another pleco gets in there space.
According to Planet Catfish Common Plecos don't like each other. It says "can be aggressive to con-specifics" I assume that means their own kind. Other people have posted in other threads to say that they have more than one Common and they are fine as long as the tank is big enough (really big) and is suitably decorated so they can hide. The Sunshine Pleco, according to Planet Catfish can reach 12". But you said size is not an issue so perhaps you have a very large tank.
Most larger plecos get more terriotrial as they grow and become pretty grumpy. Basically you need to provide enough space and hiding holes to keep all concerned content.
You sound like Alec Guiness "These aren't the droids you're looking for..."
Seriously, people are gonna ask questions when you say that, especially if you already have 3 12"+ fish The size of a tank for bottom dwellers is extremely important as one needs to allow enough floor area to keep the plecs happy.
I have two common plec's that never fisght- And as for my signiture that is spread out over 2 tanks one 126ltr and anoother 65ltr, What ones produce the most waste because i see it on my sand all the time
That's a false rumour. The fish will get stunted and as their external apearence wont seem to grow, their organs and insides will and they will eventually die and early, cruel and unnecisery death.
Common plecos get to be over 12 inches/1 foot. i really suggest you either take them back or find a new home.