The dwarf gourami is fine and a single male is perfectly fine on his own. Just keep in mind that dwarf gouramies are a little fragile so make sure to pick out a very healthy looking one from a healthy tank from a reputable LFS. Dwarf gouramies won't normaly bother anything other than other dwarf gouramies (and then it's only the males that do this chasing). Make sure you have some kind of tall plant or floating plant for him so he can set up a territory and make him feel comfortable and make sure this spot is away from strong currents.
As for your comment Durbkat on gourami aggression, you clearly did not have your gourami(es) in the right set-up. What kind of gourami did you have? Dwarf gouramies are peaceful unless over-crowded with only males being territorial and ignoring most other species of fish. Gouramies are a very wide-ranging group of fish. Just as you can get peaceful keyholes, aggressive mbuna, the dwarf rams, tiny shell-dwellers or sensitive discus when it comes to cichlids, you can get giant osphronemus gouramies, tiny sparklers, peaceful pearls, soltary, highly-territorial bettas, or aggressive paradisefish in gouramies. You can't not generalise and say 'gouramies are aggressive'. That's like saying 'birds are small (before anyone actualy decides that's true - ostrich, albatross etc)'
As for keeping angels. You don't have room for a 6" fish, first of all. Even if you did, angels don't work with guppies and, regardless of the fact that some people have managed to raise angels with neons and not get deaths, the risk of the angel eating the neons is still there and I don't like risking the fish' lives for the sake of aesthetics. besides this 'moral' reason, single angels have a tendency towards becoming aggressive and terrorize their tankmates. In a small tank with such small, very peaceful species, your fish would all likely suffer once the lone angel reached sexual maturity.