apart from different protector factors on the skin and eyes of different fishes, all these tankmates depend on their sizes,speed and whether they stay at the bottom of the tank or not(silver arowanas rarely attack bottom of your tank) for example at the time i'm keeping 1 silver arowana(15cm),2redtail dragons(14 & 6cm),1needle nose gar(12 cm),1black ghost knife(14 cm),2clown loachs12&10cm(these stay at the bottom all the time & have some kind of needles near their eyes),1Chinese High Fin Banded Shark(9cm),1catfish(8cm),2glass fish(8cm),2ramizi 6cm(fast fish) and even two 4cm jolie fish(at the bottom all time). about the ones u mentioned as much as i have seen 2 clown knifes dont do well together keep 3 or more and if ur arowana is not big enough they will kill ur arowana! about oscars..nothing is exact but i have seen oscars attacking arowanas much larger than themselves but with feeding them well and providing enough space its unlikely to face any problems..about silver dollars..i suggest tinfoil barbs instead of them cuz i have kept them with arowanas before..but if u insist u can try them! goood luck!