Greetings, salutations, Hiya!

1) There is no requirement on JvS that your backstory HAS to be written out on the boards. Most RPers accept whatever written backstory you put forward - and most characters have that backstory written out in the "bio" section of their character profile. Most characters prefer to make a few backstory posts nonetheless (and others entire backstory arcs); all you have to do is pick a place in the universe, and start writing. It is considered common courtesy to reach out to any writers that may be writing at the location, or to make a post on the planet's OOC board stating that you will be writing backstory. But, in general, as long as you indicate that you're writing in the past ("On the plains of Naboo, twenty-five years ago...") people will pretty much leave you alone and let you do your thing.

An exception to this would be if you claim any special items or materials or whatnot via your backstory. For example, one of my characters had a Sith Sword back in the day as part of his backstory. However, rules on the site don't allow you to spring such items out of thin air, so I did a small backstory arc - maybe three to four detailed posts - where I made the sword. Keep in mind only the actual acquisition / creation of those items is needed, and not an entire character arc. That said, that really only applies to certain items and materials (cortosis, phrik, beskar, crushgaunts, Sith swords to name a few) and you most likely have nothing to worry about.

2) JvS is an open universe, meaning you can pretty much post wherever you want, within reason; you more than likely, for example, shouldn't write yourself into the midst of an enemy base, currently being written by other RPers. The basic rules of the site are simply to be courteous; contact anyone who happens to be writing on the same planet you are, and leave a short note in the planet's OOC board. Otherwise, you can do what you want. Things get a little more complicated when writing with other RPers - we use something called the traffic light system which indicates what you and other RPers are looking for. So long as you have some courtesy, drop a few notices, and are mindful of others' traffic light status, you pretty much have free creative reign.