seeing as im 31 *just turned that this morning* i would wait till around level 26 - 30 then comdine and experiment.
FYI as a word of caution dont go pvp until 35-40 most of the pvpers are 40 on and 50
Definately focus mainly on one tree otherwise keeping up is going to be expensive.
What I did was start out focusing mainly on Awakened, Operative, and Kung Fu abilities. Any $info I had left went into Coding. Once I got to level 15 and could go into archives I started working on Hacker (Arbalest Path), so I could take on the archive bosses. It has worked out nicely.
The only difference I did with my second character is that I started out Akido. Martial Arts for me are a matter of preference. You don't need to worry about which Martial Arts to use as long as you are fighting npc's. The only time you need to worry about that is during PVP even then I don't think the difference is that overwhelming.
Martial arts mixed with Gunman I found really isn't a good mix. Martial Arts deals with interlock when Gunman primarily deals with staying out of interlock, so you can see the conflict. When you are outnumbered it is much easier and faster for me to even out the competition with Martial Arts than with guns.
Hope This Helps
This is more of advice for when you get to your mid to upper 20's, but you can keep this in mind while leveling.
I found that I chose to mix my martial arts up around level 25. You'll probably want to use Karate at first as it is armor piercing and seems to cause the most damage, but as you level I found it better to mix arts for certain abilities/attacks. For example, I would sacrifice my Karate Master's +5 tactics bonus and add misdirect punch for its blinding state. This seemed to work best for me as far as missioning. I would avoid some of the abilities that needed the enemy to be under a certain state and keep moves like sidekick combo and swirling ki summon so that all my abilities would be immediately usable. I found that their debuffs, especially misdirect punch and swirling ki summon would easily replace my +5 tactics bonus that the master abilities would give me. Using these abilities to debuff your opponent in the correct order along with dodge ranged clothing will make you invulnerable to ranged attacks by other npc's while taking the others down in interlock one by one.