my 50 gal has a Flash Panaque Sp. L204, White seamed starlight ancistrus L183, 4 angels, 10 pricilla tetras, 15 corys mostly paleatus, and fistfuls of various snails. they all get along real well...that being said i used to have about 150 guppy fry from before i relocated the adults and they all went bye bye in about 48 hours. So i think the rule is, unless its super fast and/or the angel has always lived with it, if it fits in the mouth, it gets eaten.
EDIT: Also add lots of plant cover and caves, and hidey places in general, it'll keep the angels from being so tempted to kill stuff all the time.