I also don't consider gourami's to be top level fish...mine thinks he owns the entire tank.
Killi's are really interesting little fish, but some species don't get along well with other fish, and they aren't easy to find at your lfs. Plus the fact that some of them don't like prepared food might make them a little frustrating for a beginner.
Hatchets really would be the easiest and most available choice. Some species don't get as large as others (I believe the sticked thread "Recommendations for a 10 gallon" mentions the smaller species), and imo getting a small group of them would be perfectly safe even if it brings you over the "inch per gallon rule" a little bit...especially considering the fact that you are looking for a fish to occupy a specific level of the tank.