You should know that inqusitor and justicar are cleric souls--not warrior--and while you can switch between the 8 souls of your calling freely due to roles, you have to use those in your calling. So if you had your heart set on playing a warrior, you can't be a inquis/justicar, you'd have to be a cleric to do that.
Inqusitor/justicar is a good solo spec, but I think it loses some in duo. A lot of your damage comes from your two primary dots, which probably won't tick for full duration with a rogue friend helping you kill stuff. It can also be made fairly durable, but that again matters less in a duo. It's not that it's a bad spec to bring to a duo, just that a lot of the things that make it good are found in soloing.
It is possible to get all 8 souls unlocked. You can have a total of 5 roles (a fifth slot was recently added). These are your preset builds. It costs money to buy roles beyond the first, with each one becoming more expensive. You'll get your second role very early in the game, probably before you hit 10. The third is easy, the fourth is feasible. The fifth role costs 100 platinum, and it will be quite a while before you are able to buy it.
I just want to reiterate that you can only use the souls from your calling (think class) in your builds and roles. A warrior can only be a combination of warrior souls and so on.