I think it's pushing it but if you were going to go with an angel regardless, the tall would be better than the long. Still, ideally you should wait for a larger tank to keep an adult or it would be stressed and may get aggressive. If this is a very small angel you may be okay until it grows a bit, but it would be better to have the tank that can keep it for life. I had to keep an angel in a 20H temporarily once and it started acting like it wanted to kill everything until I moved it out. Why not add a group of a schooling fish that you like to bring activity to the middle and upper levels instead of an angel? Maybe tetras or raspboras to focus the attention on your gouramis?