I've got a group of 4 Blue Acara in a 55 together with 3 small yellow severums, an Oscar, and a Sailfin Pleco. The tank couldn't be more peacefull.
The Acara came out of my other mixed 55 where they were constantly hassled into the plants by the other bigger, more aggresive fish. Now they're somewhere less hectic, it's a pleasure to see them out and about.
I'd say treat what you have read with a pinch of salt. There will always be individual behaviour from fish to fish, but I really don't think Acara are as aggressive as they seem to be in print.