"one setting, characters from different universes; larger scale; all OCs"
Not entirely sure those should be ; instead of , but idk.
"we keep calm amd roll with it."
Typo.
"Keep it PG-13! That doesn't include violence, mind you, but it does mean nothing sexual--and keep the cursing to a minimum, if you use it at all."
I'd personally reword that to something along the line of "keep anything overtly sexual off camera, and please keep in mind that some members dislike cursing," so forth. Something more specific that wouldn't turn away someone because they think it's too childish or something.
"Please, no homosexual relationships or activities of the sort. It'd be easier for all of us if it's not mentioned at all." Again, I'd specify on that "none on camera if at all, the religious views of some members make it a difficult to reconcile the subject even if kept solely IC."
"You might have noticed that the first line has no marker to tell you who's speaking. This is because of another important guideline--we ask you to change your title, avatar, or (preferably) both to reflect which of your characters is most important." Instead of "we ask you to" I'd say put something like "we change ours, and it would be easiest to change yours, but it's optional. If you don't, then please post a picture alongside the name in OOC and/or the character tab. If you don't change your title to a character's name, please remember to put their name before their speech even if they're the first in your post."
" or there muswt be some diminisher" 
Typo.
"So, how on Earth (or "how in Existence", as we like to say it)"
We do? Since when? lol
Put a link to the wiki in the extra information, too.