You could also do background bio's for your characters.
Where they came from
But keep adding to the bios as you use the characters. During my PBEM days I created plenty of characters and the bios evolved as the character did. And yes some of them had flaws and issues in part due to events that happened to them. In many respects is was more fun to write for those characters than the ones with fewer issues and flaws.
I would also agree with hybrid characters keeping to a minimum of two (mother and father), with one provision. If you must have a character with a multi-species i.e 4 generations ago they had a vulcan ancestor then make sure it's for a reason for mentioning it or raising it in story and not just for the sake of it.
As for plot, keep technobabble to a minimum.