Cardinal tetras do not have a long lifespan in nature; if you buy them large enough to avoid being eaten by mature angels, you are not going to have them around much longer than a year or so. Rummynose tetras, Garnet tetras (also known as Pretty Tetras), Emperor tetras, and Red and Black Phantom Tetras might be better sized/shaped for survival in your tank.
The idea of Harlequin Rasboras is also an excellent suggestion, however. From the standpoint of coloration as well as size and disposition.
Good luck.