I have the same brush with the Medium needle/head combo. I also have a Fine but it's TOO fine and tends to give me fits. It's a great brush and is capable of just about anything you want. The sister series of the gravity fed 100 brushes are capable of slightly finer lines as gravity fed brushes can use lower air pressure, but the 150 should be able to do 95% of what you want.
You should definitely thin the paint. You can buy 250ml bottles of Tamiya Thinner (X-20A) that already includes the retarder. Do not buy the little bottles or you may as well just start throwing money out the window! Or you can buy isopropyl alcohol (90% or 99%) and add the retarder (Createx or Liquitex), you need just a drop. Thinning is up to varying factors, have a read through
MusicCity's website (scroll down for his tips) on
thinning. Do note his caveat about isopropyl alcohol and flattening of gloss paints! He has lots of information there, and I suggest you take the time and read through them.
I stocked up on the Tamiya Thinner and use that, though I'm going to give iso and retarder a whirl when the time comes. What you are looking for is a 2% milk consistency. I eyeball my paint/thinner mix, but it's probably pretty close to 1 to 1. Try 15-20 psi of pressure and get up fairly close. You can practice on milk jugs or scrap styrene. Good luck!