You do know that goldfish grow between 8 and 12 inches long, right? A 22 gallon tank is way too small for 5 goldfish (even right now). They produce more ammonia than the average fish and need large quantities of water or at least large frequent water changes until you can find a more permanent solution.
Your goldfish should not be sleeping up side down (I can only assume this is what you mean as you haven't described exactly what "playing possum" is). If you have fancy goldfish this is a sign of a swim bladder or digestive problem. Often times this is caused by poor water quality and poor quality food. In order to improve water quality, you will need a larger tank.
On the subject of the plakat, he should not be in with goldfish. While goldfish can handle temps up to 90F, they shouldn't be in water warmer than 80F long term. Warmer water speeds up their metabolism and causes them to release more ammonia. They also need to be fed more at warmer temps to maintain a good weight; and of course more food in means more waste out. As your goldfish grow, the plakat will become food. Despite their bulbous bodies, even fancy goldfish can move rather quickly when they are hungry. A fully grown goldfish could swallow a plakat whole.