Regarding self inserts.

I'm aware that Butt Monkeys and Self Inserts that are parodies usually get a free pass, but what of regular characters that play a role (not central or even main, but not Spear Carriers or cameos either).

I was considering making a soldier based off how I act/my antics on TV Tropes [yes, the novel in question is a pretty bad case of Write Who You Know], doesn't do anything particularly major, but is nonetheless a common character.

Sounds awesome in my opinion. Go for that shite, meign.

That is a Creator Cameo, I believe, and totally harmless. Go for it.

Well, in my story (the one in my sig) the main character is a reverse self-insert. He thinks totally opposite of how I think (most of the time), and does the last thing I would do in any situation (usually, this is "resort to violence"). The real self-insert plays a smaller role than he does, but still pretty up there basically all of my good traits (intelligence, enginuity, sense of humor, Large Ham) and bad traits (laziness, stubbornness, pride, love of bad puns) taken Up To Eleven.