Hi, few things to say...
1 The term for self-insertion is Mary Sue/Gary Stu depending on your gender. Frankly, because of the type of non-active person I've been, I always came up with my own self-character for insertion in various fandom franchises. The caution is not to make your self-character uber-empowered that he or she can just wipe away all problems with a flick of the finger. Knowledgeable or brainy, sure; strong, if you want; speedily quick, that's your call; just so long as they have some normal flaw like everyone else. Which is why I'm generally okay with self-insertion of ourselves into Muppet-related fics or the creation of other original Muppetlike characters.
2 If you're going to have characters, or in this case other people's self-characters involved in your fic, ask permission from that other person first. I've done it myself on occasion and I'm thankful that my reputation here has ended up in positive permission most of the times I've asked to bring others' characters into my own realm.
3 They can get away with all their cruddy posts in the fanfic section because they can. Basically, we need assistant moderators who can either alert or help tidy up the place and we need to give full staff moderators the power to not just ban members at whatever point they become spammerish; but to pull off the magic trick of automatically deleting the offending parties' posts when their account has been banned as well. I'm doing all I can, but I'm just one person... *Song cue? Have two hands only. Not to mention the jerks don't learn anything from having their posts deleted, posting the deleted garbage over and over again.
As far as PG-13 fics, that's a tricky subject. So far the few ones I've seen posted here are by authors I have full respect in, namely Ruahnna and newsmanfan. If anything clearly crosses the line past MC's general feeling, contact a mod so they can look at it and render their own decision.
Thanks for your support, have a good day.