Some good advice been given here. Congo tetras and rainbowfish would be ideal companions for Ctenolucius hujeta, as would silver dollars and medium-sized barbs such as clown barbs. If your water chemistry was sufficiently hard and alkaline, there are some livebearer options too; I keep mine with Ameca splendens but swordtails, as mentioned, would be a good size though both these require water at the cooler end of the range, so factor that into your community. I'd avoid mollies though; adding salt isn't really an option here, and while mollies can do well in hard freshwater, they often don't, so I like to keep the "slightly brackish" option available when keeping mollies, even if I start off with them in freshwater conditions.
Among catfish, almost anything will work with Ctenolucius hujeta, even reasonably large Corydoras. Obvious choices include Brochis splendens and Synodontis nigriventris, both of which are the right size, gregarious, and lots of fun. Having said this, spiny eels are difficult to feed, and unless your specimens are already feeding properly, don't add any catfish at all, except perhaps something that won't compete, like an Ancistrus species.
You don't want hyperactive or territorial fish in this tank. Both spiny eels and Ctenolucius hujeta prefer placid tankmates. Right now I'm trying a Mogurnda goby and an Anostomus with my Ctenolucius hujeta. The goby seems to work extremely well, but the Anostomus is occasionally nippy, though this can be reduced by providing more green food for it to nibble on between meals. Both bring colour and activity to the aquarium. They should be fine with spiny eels that are feeding well, but I wouldn't add them until you were happy your spiny eel was feeding and putting on weight.
Cheers, Neale
You are really going to irritate people if you keep making millions of posts, seriously... people wont bother replying.... make one post and stick to it, you can reply to your own post with more questions.... 
