5 plattys >>>>> >>>>Not good, they like cooler water
2 lace gouramis >>>>> Notorious for harassing Discus
1 dwarf gourami >>>> Can work, rarely though, watch for fin nipping
2 dutch rams >>>>>> Usually do well in same tank as Discus
5 neon tetra`s >>>>> They like cooler water, wont last more than a few months in Discus water
1 male fighter >>>>> Will be ok with the heat but another notorious fin nipper
1 female fighter >>>> Shouldbe ok. Maybe.
Basically thats a bad mix for Discus, Discus need temperatures above 29C or 84F to do really well. They are not community fish as you can gather from the replies above.
A single 3" fish will be miserable and shy on its own. It may eventually retreat to the back of the tank and stay there. If its not feeding now, thats a bad sign, they will go on hunger strike if they aren't happy. Have you noticed any of the symptoms elgecko described?
Many Discus are fed tubifex worms at the hatcheries and so wont recognise flakes and pellets as fod, they may take frozen bloodworm and brineshrimp, but you should try and wean them off it as its not a balanced diet. After a while they will realise that the flakes etc are food from watching the other fish.
Top priority now is to get the little guy a friend or three of the same size, I would try and bring the platties, gouramis and neons to the shop to see if you can get a trade (but ask before bagging the fish!) or buy a second tank, 25-30G would be plenty for them.
They can get over 10" if you use RO water with the right additives so be warned!
Ken