Waay less plecos, maybe one of the fancier ones that stay small if you really want one. Commons get huge, and you'd have to build a pond in the future for the arowana and redtail.
i am also confused with the number of plecos... however what everyone else has mentioned is very true, in my 125 i have a large oscar with an almost full grown JD, they get along fine, i used to have a 10 inch jag in with them but my oscar destroyed him along with my 9 inch green terror over night R.I.P(beautiful fish).. The flowerhorn could easily have one of there massacre's with this list, but when its all said and done, maybe they will get along fine, as long as the RTC, Arowana and the pleco army are not part of the mix
ur tank is overstocked. first of all you'll definately need to remove the rTC cuz it's aggresive and you'll need a 500 gal tank for that beast once it matures. also, flowerhorns are very vicious. another thing is you put too many plecos in the tank. Cmmon plecos and grow well over a feet. The silver aro you have will grow to more than 3 feet so you'll need a 200 gal tank.