I'd add a few more kuhlis first off, as they really like company.
Then I'd go with the congos, myself. They get rather large (about 3" long and they are thick-bodied) and really are active nice colored fish once they are settled in.
With angelfish you may end up with a pair, and from what I've read they can be really nasty when they breed. If you don't end up with a pair they may squabble quite a bit. You may could do a single one with a group of 6 congos though.