Right, Firstly, DO NOT put either Neons or Cardinals in with your angels. the reason 'some people' say you can't do it is because you can't. They might be ok for a week, or a month but basically they're swimming fish food. The angels will have them, it's only a question of when.
My angels, which I have had since they were tiny (1 cm across) grew up in a tank with a shoal of neons and they were fine for months. Then all the neons disappeared overnight. Don't do it.
Secondly, you need a 2 or 3 more Corys, they're social fish and are only happy in groups. 5 would be ideal.
Don't put a Betta in there either.
If you still have room, I think these cichlids would be ideal for your tank
http
/fish.mongabay.com/species/Anomalochromis_thomasi.html
Also known as the African Butterfly Cichlid, peaceful, colorful and interesting. I have 4 in my 45GAL with both angels, corys, a clown plec and others with no problems.
They seem to be happy to live in groups but I haven't had them long enough to be sure of that.