Adult Angels can eat fish the size of large Neon Tetras, so this would include most of the small community fish like Guppies, Danios and many Tetras. Some are not so predatory, but think also of the small fish, constantly living with the worry of a large fish that could eat them!
Angles are soft water fish and thrive in very warm water, so select tank mates that prefer these conditions, too.
Congo Tetras, Brochis splendens, Anostomus, Yoyo and Clown Loaches are often mentioned, but these are high water flow fish. Quite a few Loricariads, but look into Farlowella, and Sturisoma for some 'different' sorts.
Neon Dwarf Rainbows, but caution: The males get really pushy toward the females, and can harass them to death. Get at least 3F:1M.
Other Rainbows can work, too. Many come from waters that change a lot seasonally, so can handle a wide range of conditions. Boesemani, Parkinsoni and others will work with Angels.
Moonlight or Pearl Gouramis are also possibilities. Beware: most Gouramis are aggressive and territorial, though Pearls are often a bit more peaceful. Most of the time their aggression is directed toward other Anabantoids.
I have usually had problems with more than one mated pair of Angels in any tank 4' long or shorter.
By the time the tank reaches 6' the Angels quit trying to claim it all, so 2 pair can work, with lots of plants and driftwood to mark territories. Almost split the tank in half with the plants.