Not much information is available on some of the specifics of the rank structure, especially when it comes to the dress uniforms. Furthermore, there are some inconsistencies throughout the films:
Captain Deladier inaccurately has the same standard uniform rank insignia as Zander (she's a Captain and he's a Lieutenant, both in Fleet). However given that Zander's rank equivalent in the Mobile Infantry would be Captain, someone in the crew probably got confused and gave Deladier the Mobile Infantry Captain rank instead of the Fleet Captain rank. Her rank equivalent in the Mobile Infantry would be that of Colonel.
Zander and Carmen share the same rank insignia on their dress uniforms (Lieutenant), however on their standard uniforms Zander is a rank above Carmen (he has the lightning bolt with two lines through it and hers only has one line). The most logical explanation is that Zander is a Lieutenant and Carmen is a Lieutenant Junior Grade, since they're both Lieutenants but he is a rank higher. This would also be consistent with real-world Navy ranks.
The dress uniform rank insignia of Captain for Fleet differs a bit from the dress insignia of Colonel for Military Intelligence, despite being equivalent ranks. This can be seen when looking at Carl's Colonel dress insignia compared to Captain Deladier's and Carmen's Captain dress insignia, Carl's seems to have more areas grayed out on his indicating a higher rank.
The dress uniform rank insignia for Lieutenant is incorrectly used for the Sky Marshal's shoulder epaulet in Starship Troopers 3: Marauder.
It is unclear how rank equates to billet. Ex: When promoted to "Corporal" (a junior NCO rank), Johnny claims that he needs a 'squad leader' under him, and promote Dizzy to the task. However, in most conventional terms, a 'squad leader' is at least the rank of sergeant, and has a corporal under them as an assistant squad leader.