I've never kept that particular species, but you may be able to squeeze a pair into a 40 gallon, I find that most of the time, the size of tank needed with cichlids refers to how much territory they need rather than the actual size of tank, so a Male/Female pair would probably be ok.
As for tank mates, assuming your water is suitable for Acara's, being around pH 7 and low GH, then most peaceful tetras would make a great option. Cardinal Tetras are very beautiful, as well as fish like Diamond Tetras, which are a personal favourite. If you are using fine sand in the tank, then Corydoras sp. also make a good option. If you're looking for something a bit more strange, then Hatchetfish can also work, as long as you have a tight fitting lid, as they can jump.
If you wanted to do GBR, then I would say you could probably get between 3 and 6. Also, keep more females than males, preferably a ratio of 1m-2f. This will help with aggression a bit. With GBR's your options are slightly more limited as, like I said, they require higher temperatures. Cardinal Tetras again are good, as they can tolerate these temperatures, as well as Corydoras Sterbai specifically.
But before you set your heart on anything, make sure you get your water test done, as none of these fish do well in hard water, so if you have it, you may need a different plan. Also, what are the dimensions of your tank?
