I would think 4 Angels would fit comfortably in a 55 but beware if spawning occurs. I almost lost an adult due to fights and liplocks over a female...I wound up mvoing the weaker angel to another tank.
I disagree with fanman on the topic of the RTBS harassing the cories. I've kept them together for years and have never had a single problem. Since your tank will be planted, the cories will have plent of hiding spots if something does happen, but I doubt it will. Providing the RTBS with it's cave should keep him satisfied.