As for the eels, if you are planning a more aggressive tank, then they will do just fine. But if you are planning a community tank with smaller, more colorful fish, they will start to disappear. I have 2 fire eels over a foot long, and all the fish in the tank are more aggressive fish like sharks, tiger barbs, etc. And the eels do just fine with loaches as well, so I have loaches for my bottom dwellers. And finally, do research on the types of fish. I have my fire eels in a 55G, but within the next year, I will have to upgrade to a 125G minimum for them. But I'm already planning either a 265G or a 300G for them anyways. Some eels stay small, about a foot or a little more, but eels like my fire eels have the capability to grow 2 1/2 to 3 feet, and their max size listed an several sites is 40 inches.
55G Filstar XP3 - 16" Fire Eel
75G Medium Planted - Filstar XP3 Low Light - Established Feb 2006