the Veja species aren't really known for their agression with a few exceptions, they are more known for their colors and size!!!! maculatas(sp) common name is black belts are known to be a bit agressive every now and then. but I don't think that I have heard much about syns being that agressive. but like I said they are going to need a large tank and the acaras might not be able to handle a full grown male.
as a side note I saw some pics of an 11" male, so definatly going to need a large tank for it and just keep an eye on it if you notice any agression that looks like it is going to kill any fish in your tank seperate it right away