angels are possible, but with risks.
Firstly - a random 2 may not get along, with the weakest becoming the victim of bullying. Personally I would go for siz juvi angels (this is exactly what I have in a 4ft, journal in my sig) as then the aggression is spread. Mine are constantly squaring off to each other, but nothing serious ever occurs.
Secondly, if you buy them young they MAY grow up not seeing the neons as food. This IS a risk as neons are natural prey to angels.....but again, this has worked for me. I keep 15 in with mine.
Discus - quite particular about their water and like a higher temp than most Trops can handle long term (27c -30c).
As for any other medium to large sized cichlid, if they can fit their tankamtes in their mouths then there's a strong possibility that they will try to.