Loaches grow very slowly. I'm suspecting your loaches are clowns. It takes several years for them to reach full size. I am rpesonaly against keeping them in a small tank based on this but a lot of people still keep them and plan to 'upgrade' in future (which most neevr do and end up searching for a home that will take in a couple of 10" loaches). Pacus, on the other hand, grow very quickly and grow larger than the loaches. If you don't have a 200 gallon tank, re-home it now, while it's still small, and your LFS can take it back.
As for stunting. There is no doubt that a form of 'stunting' exists in the wild which limmits the size to which fish can grow in crowded conditions. However, this is only a temporary measure for short periods of time. It is an adaptation meant to save fish' lives when conditions are less than dieal. It is not meant to be a long-term thing and is only slight in the wild. I have done a little research on stunting and it appears that the fish release some kind of growth inhibiting hormone and that's what limmits their growth. In a tank where you don't do water changes, this will probably limmit the fish' growth in a similar manner. However, it would probably be a lot more severe (not to mention all the side effects of not doing water changes!). Some people believe that when a fish is 'stunted', the organs inside continue to grow while the outside stops. You can obviously imagine what that would be like - suffering as the organs are crushed until the fish dies a slow, painful death. I have not found any actual evidence to support this idea. Still, there is no doubt that purposefuly stunting a fish is cruel and harmful. If you do stick to a good maintainance outine, there is absolutely nor eason for a fish' growth to be stunted - even in a very inadequately sized tank. It will eventualy out-grow its home.
I should add that what I've just typed is based on the research I have done on the internet. I have read a few papers about stunting in wild populations of various food fish species and obviously also from my own experiences and what I have heard from others. I may not be completely correct. Regardless, there is no doubt that you should provide a fish, of whatever species, with a tank large enough so that it can grow to its full potential, live a full life, thrive and prosper. Doing otherwise is, IMO, completely unfair for the fish and, in some cases, down right cruel. (Please don't misunderstand me. I do appreciate that some people don't realise the size fish get to or their requirements etc or that they are mis-lead by other people. That's fine as long as they are prepaired to do something about it once they've realised their mistake.)
or does it only apply to some fishes and not others? 
