I'd say a Zig Zag eel that tops out at about 6" is your best bet. You could also do a peacock that will likely get to 10-12" if you want something bigger. I'd so soft substrate, or fine gravel, as they will sometimes bury themselves. Give plenty of places to hide, otherwise they will jump. I recommend PVC pipe, driftwood, etc. As for tank mates, do not put anything small enough to fit in their mouths. As long as you don't go with smaller than a neon tetra you'll be fine, although some people keep them together with no problem. Overall, I'd say go with a Zig Zag.