An 18" wide tank is way too small for tinfoils, and even the smaller red tailed tinfoils. Also they're shoaling fish, so they should be kept in a group of at least 6, preferably more.
Keeping 2 of any shoaling fish is as bad as keeping 1, because will usually get bullied by the other. The 'pecking order' thing is just because they're not in sufficient numbers.
Also your festae will want space as it grows, and I can't see it living with other fish in such a relatively small tank when it's big.
Actually I wouldn't confine any fish that cannot defend themselves, like tinfoils or any of the dollars, with aggressive cichlids at all. It may work for the cichlids, but it's constant stress for the others. I will even go as far as to call it cruel.