I figured I would get all of my most newbish questions into one post
1. Is it better to try to get a single character high enough to get Legacy bonuses before starting Alts, or is purely preference? ( I am an Altoholic)
Get to the end of Chapter 1 with a character to start a legacy, then it doesn't matter.
2. Can you get to 50 without doing Heroics/FP/Warzones or will it be a major grind? ( I don't mind grouping, but being familiar with Raiding I am also familiar with some groups reactions to newbs)
Definitely. You may need to throw in a couple space missions if you're finding yourself behind but it's not a big deal. You should also do the bonus quest series on the planets. I have a feeling that most casual people tend to do it this way with a few flashpoints thrown in.
3. Op/Sniper; Is it better to go Lethality or Marksmanship?
I'm assuming you meant agent/sniper...
Engineering or marksman for PVE leveling.
End game PVE either works.
For PVP, lethality or engineering are best.