Celestial Dragons represent 19 Royal Families (Not counting Nefertari)
Each Family is like a Kingdom of it's own
For example, if Riku & Nefertari & Neptune Family made an Alliance, Heads of Families would be Equal in Authority compared to each other, but They can give Orders to their own Family, Neptune can't give Orders to Riku or Rebecca, but He can give Orders to his Children for ex,
So each Family from Celestial Dragons should have a Leader, who's like a King, and his Children would be his Second in Command, or They could Succeed him, similar to Chinjao who was Succeeded by Sai who in turn also have Boo as his 2nd In Command
Since these Families are Equal in Power Overall, their only way to compete with each other is when organizing Games or a Hunt for something or an Objective, and so They need some Members to represent the whole Family, that's who Holy Knights are, They are like Champions of each Family (Head of Family can also be a Holy Knight if They want)
And since it's a Competition, then Best of them gets to be their Leader/Captain, making him not only represent his own Family, but entire Mary Geoise Kingdom, that's why it's Garling who handled Mjosgard's Execution for example,
For some reason, these Families decided to secretly bend the knee to Imu, and if not for him, Gorosei don't really need to exist, Heads of Families would simply be Supreme Leaders of the World, but it's implied that Imu's Power involves making Five Avatars, so He chooses Five of those Godheads to be above Rest (As seen when Garling was chosen)
So from what i understood so far, this is Hierarchy of Celestial Dragons:
1. Imu --- Supreme Authority
2. Five Elders --- Only Answer to Imu
3. Rest of Family Kings --- Only Answer to Gorosei + Imu
4. Rest of Family Princes --- Only Answer to their Own Parents & Gorosei + Imu
5. Leader of Holy Knights --- Only Answer to Family Kings & Gorosei + Imu
6. Rest of Holy Knights
7. Rest of Celestial Dragons
Then comes Non-CD Members such as Cipher-Pol, Marines & Jailers, each have their own Hierarchy,