Eco complete should be fine for them. I keep A. Agasizzii Double Reds and they are awesome. The Cacatuoides Triple Reds will be amazing too.
In a 55 you should be able two or three pairs. Just be sure to provide lots of structure and hardscape to break up sight-lines.
Also, if you wanted more, you could put maybe two males and like, four or five females (someone correct me if that is too many females) to create the harem colonies that alot of apistos are found in. This will also help disperse agression between the two males (less competition for females), and it will also suppress agression towards any one female as there are more for the male to pick from.
Generally speaking though, Apistos are not hard to keep, but they do like good water qualities.
Keep us filled in on how it goes.
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Ehei-Pimpfen 276 yo!