If the shop will take extras back get six juveniles.
They will pair off naturally once mature. Once the pairs are formed take the extra fish back to the shop.
Six will give you a good chance of having at least 2 males and 2 females. If you get a third pair out of them, it will have a higher value than two fish, and the shop might take it back more willingly.
Unfortunately, my LFS has a tank full of "Assorted South American Cichlids" and even on asking the staff have no idea what is what. I could see at least what appeared to be firemouth in there and some other similar looking fish with pale stripes and spots on the gill plates. They told me that they could maybe try to find some for me and order them but they didn't really know. I'm not sure if I'd end up with firemouths. The online shop I found that carries them as a regular part of their stock is unfortunately on the other side of the country... I don't think I'd have room for 6, I might be pushing it with four. I wonder if I could just sell the other two to the LFS... maybe worth exploring. I don't plan on fish until early March if I had to guess, it will depend on how well I can get these plants to do. I want to run low tech so I'm guessing it will take longer.
A piece of wood, some rocks even plants can be used as territory separators, as long as the fish have a clear indicator of where the territories meet.
I have another piece of bogwood I took out this afternoon which might serve me well here if I can figure out a way to position it so it doesn't look exactly like that is what I'm doing. Plus the rocks I have already boiled and I have many more I can sort through for options. I need to find some decent pictures of some Thorichthys habitats that aren't focusing in on the fish.
I'm so glad I have something in the tank now even if it's just plants. It's so much nicer to look at.