I would actually go with 5 angels as a minimum number.
If you get three, most likely two will pair off an pick on the third (Although i know some ppl who do this simply because then you get two completely stunning angels, and one well....scapegoat angel) So 5 as a minimum usually prevents angels from pairing off.
I beleive ghosts knifes can become agressive as tehy grow larger and they also can grow really big (eventually too large for your tank IMO)
IF you are looking at silver dollars, remember that they are vegetarian, and usually consume all plants in the tank, so if you want plants...hehe its best to stay away from silver dollars.
For other tankmates... tetras, rasboras, rainbows (probably the dwarf neon rainbows) would all be nice schooling fish to offset your angels as centrepieces. (Just make sure that the tetras, rasboras are large enough. Adult angels will eat small neons, or cardinals. Also for tetras you might want to stay away from red eye, black widow, serpae and colombian as i beleive they have been known to nip fins.)
Cories for cleaners, and otos for algae.
You can try rams but if they pair off then they will become aggressive. I would stay away from kribs as they are way too aggressive IMO
Angel fish are actually quite similar to discus in many ways. Any tank mate suitable for discus will most likely be suitable for angels.