looks good, heaps of rocks! if you can anchor plants using rocks then i'm sure an eel won't cause you too much fuss (aside from being fussy eaters). try and give it lots of hiding places above ground level.
a peacock eel would be the best choice for your size tank. the tyre track and fire eels can grow to almost a metre, while a peacock will max out at around 15-20cm and would be happy enough in a four foot tank.
i agree, barbs are awesome. just pick some that are too big to become a pantadon snack.