You won't make money by breeding fish. Sorry to say. I've been breeding different species of snakes & lizards for years, and it's hard to actually make a profit. At best you make enough to cover the cost of care, but it's hard to even do that sometimes. But, go ahead and try.
Anyway, as they've already said, the tank is too small for fire eels. It's too small for ONE fire eel, let alone three. But, that's not the real problem- the real problem is that those fire eels will kill each other. They get territorial as they grow, and everyone reports it happening at different sizes. I've heard of 8" fire eels killing each other. I'd recommend replacing the fire eels with peacock eels- they actually seem to enjoy each others company.
After that, a palmas may work. But you'll need to watch it with spiny eels of any type- i've heard a couple people say their bichirs and eels attacked each other, but it seems pretty rare.
