Hammertime, I was just using River and Simon as an example.

How about findin a Beta Reader? I guess when I'm done I can just post a request for one.

Belasera is right!

The beauty of fanfic is that you take the characters and write what you want. Hell, if you really felt like it, you could have Jayne start life out as a girl and have a major life change in his mid twenties before he joined the ship (not that I'm saying I'd read that fic, 'cause I probably wouldn't, but it's your fic).

You can set it anywhere and anytime.

You don't have to use Chinese if you don't want, it actually makes up very little of the Firefly dialogue and a lot of really good fics don't have any.

The only real rule is: write whatever you feel comfortable writing.

As for choosing titles, I can't tell you how everyone else does it, but what I usually do is write the scenes that are in my head first, let the fic flow and usually something in that will spark off a title. Whether it be a line of dialogue, or a theme that runs through it, or just something that occurs while I'm writing.

Hope that helps.

You have to...nothing!
That's the beauty, you can write whatever you want, add or take away whatever you want and use as much or as little chinese as you want.
It is totally up to you how 'canon' you want your fic, that is how closely it will resemble Joss's story.
I find it helpful to write in 'voice', but you don't have to if you aren't comfortable with that. Especially if the Tams are your focus.
(ooh, what about a dead twin for River?)
Just enjoy whatever you write!