I've reached the point where I have three mission choices, the guy from the cafe, the hit from the safehouse computer, and the hit from the Troll philanthropist. Does the order that I do them matter?
Talk to the guy in the Cafe first...
The mission he gives you is a trial run. If you complete it he will give you extra instructions for the other quests. If you leave him until later you can't get the trophy associated with his organization.
This isn't required, but it gives you more options as the game progresses.
The game lets you do every mission in the game and gives you warning when you are about to abandon them for the main plot line.