Whats been said about the RD is right, i did it once and never again, as for the blue acara NO! you must remember an oscar grows soooo fast were as the acara doesnt, my oscar and acara got on well, untill it started to grow, my acara was stuck to one corner of the tank untill i took him out, i have a blood parrot with mine atm as well as a convict and black ghost knife, i chose the parrot because its a strong as a RD but as peaceful as the severum, when they fight none of them back down untill i look at them and there attention turns on me, but they have never hurt each other......
PS: !!!!!!!! Do not put an oscar in a 10g tank even for a month for the simple reason, these fish pollute (amonia) and alot of it, amonia stunts groth and will kill him and any other cichlid or fish you have, he will out grow it as well in no time soon, remember fish out grow there tanks no tank will stunt a fish's growth unless there is amonia in it.
all those fish in my tank are in a 55g and im thinking of going bigger..(only cause i want to add another oscar)