in a 55 i would be careful and keep an eye on the convict, acaras are tough for there size but no where near the toughness of a con.
i would be more concerned for the fm though, they dont grow as fast as the other 2 and arent as tough as them either, i would make sure the fm was a little bigger if i were doing this set up.
let the fm dominate early on or its likely to get picked on by the acara and con, the con will likely take over in time but by then the fm will be better equipped to look after itself. or keep a close eye on them if you get them at the same size.
if i knew then what ive gained from experience now, and it were my setup, i would add a slightly larger fm a couple of weeks before the other 2 and add the other 2 together once the fm has settled in.
this is just from my experience of the 3 fish species you mentioned though so you may have a totally different one.