Some movies have swears like the ones you mentioned, but are only rated PG. What it comes down to is the severity of such words and the frequency of their usage. If only one or two such words are used only once or twice, a PG rating should be okay. If the words appear more frequently or are more extreme, go with a PG-13 rating. Anything beyond that is likely to warrant an R rating.

As long as you rate it as PG-13, you should be good to go.

That is what the rating on your fanfics are for. I have a fanfic with swearing in it. I rated it high so a warning comes up when some people try and read it and it says what age the fanfic is for.

Just as what the webmaster says, Animation Source is a "Kid Friendly Site". I don't think swears in public can be tolerated. If you want to make finishing touches to your story go to the story you submitted and click FanFiction not ready. If you story does contain words like that I suggest you rating it as PG-13