That is a great looking setup that you posted. Tanks like that are right up my alley.
I personally think that guy's tank was a little small for a group of FM's , and I wonder how his fish got along once breeding started.
I would recommend a slightly smaller species than firemouth if attempting something like that in your 40. Amatitlania nanoluteus would be perfect. 1 male with 3-4 females is what I would go for. Any more than one male and you are bound to have problems, most likely one male being killed.
There are some other cryptoheros available right now which are very cool. Jeff rapps has septemfasciatus, I saw a few from his stock in person a couple weeks ago, and they are very pretty. Myrnae would work as well. I would just stay away from convicts or hrp's. They get pretty nasty.
As far as tankmates I would go for some livebearers of your choice. Any of them will work. I avoid the "fake color" bright red/yellow ones you see in the LFS when trying to do a biotope. Swordtails are a good choice and easy to find in "natural" colors.
As far as catfish, there are very few cats found in CA available in the trade. A close choice is the bristlenose ancistrus pleco, as ancistrus can be found in panama (not common bristlenose- but it's about as close as you will get)
Here is a tank I've had for the last couple years with a similar setup- P. Panamensis group, single female H. tamasopoensis, ameca splendens/belenosox belizanus livebearers, ancistrus. This was a 5' tank.
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