1. From what I've seen, it's considered polite to ask first, but not essential. Also, don't be concerned if they don't friend you back; usually it just means they don't care to have your entries come up in the same interface as their preferred set, or else they don't want you to see their locked posts.
   
   2. It is entirely normal and permissible. I know of more rec-and-link communities for fanfic than for original fic, but that's probably because I look more for the former.
   
   3. Unless you become particularly incoherent or rude, odds are you won't be actually asked to shut up. That said, it's preferred to have entries beyond "a certain length", or containing several images, behind a cut. That certain length is fairly ill-defined; as a rule of thumb, I'd place the cutoff at two decent-sized paragraphs, or an equivalent length of shorter ones.
   
   4. This is where I pimp Milliways, right?
   
   5. Ansela's already answered this one, so I'll just reinforce a couple of points. Remember the quotation marks. Remember the dash ("" as opposed to ""). Conversely, if you want to insert the username of another user or a community, leave out the dash.

Friending: my usual rule is 'ask first', especially if the person in question also posts personal/RL stuff to the same journal as their fics. A lot of people don't like being randomly friended, especially if they have deeply personal stuff in their journal. If it's fandom stuff you want an alert on, see if they post to a comm and join that to get updates.

There's no rule against posting original fic to LJ. It's not exclusively a fanfic archive a la ffn after all. There are many, many things on LJ that aren't in any way connected to fandom. The only problem would be if you tried to post original fic to a fandom comm, as people wouldn't appreciate that.

As for the LJ-cut... Do Not Use The Rich Text Editor. Seriously, it's more trouble than it's worth and it's a lot easier to hand-code everything than rely on the RTE to not eff up. The code for an lj-cut is [lj-cut text="Blah blah blah"]Hidden text goes here[/lj-cut], replacing the [] with . The speech/quotation marks are *essential* for cut text of longer than one word. The amount of times I've messed up and forgot to close them and not been able to figure out why things weren't working the way I wanted them...