Well i have heard many account of rams nipping at the fins of the slower moving angels. I have 2 German Blue (Microgeophagus ramirezi) and one angel in a 45 gallon, and as of yet have not witnessed any problems.
Understand too that a each species gets older, they will become more teritorial, and 20 gallons to go around is'nt going to be a lot of room.
Which ram were you thinking of getting, Microgeophagus ramirezi (German Blue Ram) , or Microgeophagus altispinosa (Bolivian Ram)?