Looking for a certain type of FW fish

I saw some ropefish at petco, they look cool, but I don't know about size requirements.

They get reasonably large (24"+), so a 40g wide is pretty much a minimum.
 
the LFS around my place only have dojo loaches and peacock eels. how would a peacock eel work in my 20? would the 20g be to small?
I saw a needlefish today, that fish is cool, its a shame my tank is too small to keep him.
I guess I could order the kohli loaches online if the peacock eel will get to big.
 
AquariaCentral.com