ok a couple of things i want to mention -
1 - silver dollars are schooling fish and very nervous at best they really need to be in a small school of about 6 to be comfortable
2 - being that silver dollars are fast fish certainly quicker than a gourami a healthy silver dollar would not sit there and allow your gouramies to beat on him
3- the gouramies are prob doing this because they notice they fish is weaker than them because it may not be a healthy fish -either from stress, disease, water conditions etc
i had this happen to an angel fish once - he was fine living with everyone then all of the sudden fish started kissing his sides he lost his slime coat, color, appetite and was dead within 72 hrs -
isolate your SD either to a new tank or with a divider n then use melafix but make sure there is adequate air flow - (air stone or good filtration)
goodluck