House style for military character names when credits do not give them

Well, we start with https://help.imdb.com/article/contribution/filmography-credits/characters/GKLQVXKH7U8DATWX#, to get a general idea, but as we can see the guidelines do not account for all circumstances.

If a character's given name and family name are revealed (directly or indirectly) in the movie, then he/she should be listed by name without any titles, salutations or embellishments, especially if the character has been or will be promoted or demoted to another salutation or none at all. If a character is known only by a pairing of title and a single name (forename or surname, or just "name"), then we might want to shrink the title down to just one word or an abbreviation, so "Staff Sergeant Hunt" should probably be shortened to either "Sergeant Hunt" or "SSgt Hunt", the latter if it is important to convey precision. Since the example only amounts to three names/words, "Staff Sergeant Hunt" might also be fine. If there is no ambiguity, then just "Hunt" would be fine too. We should try to keep it as consistent (throughout a given character list) as possible, whatever the case. There is no need to abbreviate "General", "Admiral", "Colonel", "Major", "Captain", "Commander", "Lieutenant", "Sergeant", "Ensign", "Corporal" or "Private", "Seaman" or "Airman". Salutations like "Master Chief Petty Officer" would best be abbreviated, especially if a distinction among MCPO, MCPON and MCPOCG is to be made.

The aforementioned is all in regards to movies that do not identify the character names or actors' roles in the on-screen credits.

Given the fact that IMDb has an international audience, I would strongly advice against the usage of abbreviations like col or sgt because it is safe to assume not everybody knows what it means. And in the case of col, one might think it's someone's name instead of someone's rank.