How come whenever I get an officer candidate, they disappear if i dont click enlist? I'll go to set up their costume or whatever and somehow they disappear if i end up selecting one of my existing officers.

Must be some kind of odd bug. I've never encountered it myself so sorry I can't offer too much help.

are you useing them to train skills?

Yep sounds like he's training skills, since he mentions selecting officers he has already, but he does mention setting up uniforms, and I don't think that window comes up if you use them to train.

We need more specifics on exactly what you're doing.

Seems like you're training skills instead as previously mentioned enlist the BO. I can tell you that either the new Officer joins your crew or the one trains you a new skill. Maybe it opens a new window ingame or a new screen is being loaded (during combat), when you attempt the dialog box of the redshirt. Or you aren't able to that a new crewmen joins your crew?