Dharma Protectors and Yidams have completely different functions and they are usually the same beings.
Yidams are for you to identify with and become attained through their practice.
Dharma protectors serve to protect you while you are practicing the yidam.
When you start the practice of the yidam, you certainly cannot gain the benefits in one day or one week or one year. It is a gradual process.
During the process, many negative karmas manifest to attract situations that might throw you off the path, the dharma protectors help you stay on the path.
Do not be confused. Take one yidam and one protector and you will be fine.