Here are the rules to certain races
Shinigami: There are only gonna be 2 maybe 3 captains all the time including the head captain. There isn't just one lieutenant for each captain there can be many lieutenants, But all lieutenants follow all captains. Same for third seat, fourth seat, so forth. If you make captain rank but there are no captain spots open you may become captain level lieutenant, But you still follow the captains. To learn shikai you must do a 1000 word training post for bankai you must do a training post of at least 1500 words and you must be captain ranked.
Arrancar: There is going to be one leader and then 2 espada maybe 3. Other than that you will be ranked as either normal Arrancar, Vasto Lorde, or Adjuchas. Once you become Arrancar you will need to do a training post of 1200 words to get your Ressureccion, Also only the Segunda Etapa and the leader can learn bankai which will be a 2000 word training post.
Human/fullbringer: There will be one leader then you will be ranked class 2 class 3 or class 4. The power of your fullbring depends on your class and you can develop more powers for it as your class increases. To master the first level of your fullbring you need to do 600 word training post, To evolve it further it will be 1500 words.
Quincy: Same as Human/fullbringer