I agree, the Koi angels are great. Just watch out if you get a bunch of them...you can't tell them apart, and you lose the appreciation for the individual personalities that is so intersting in keeping cichlids.
I love your tank. I've been trying to decide what to do with my new 150G tank and I think I will try something similar to what you have done. Some Angels with Tetras and Cories.
I like the look of the darker substrate as well. I want to do sand, but the Eco-Complete looks quite nice.