To the "surface injection" questions: This is not explained satisfyingly in the manuals.
Fluent injects one particle in each numerical cell that shares a face with the surface you chose. So let's say your velocity inlet has 1200 boundary cells - then Fluent injects 1200 particles.
Now, in the "Discrete Phase Model" dialog box, you will set the "Inject particles at ..." e.g. "Fluid Flow Time Step" - meaning Fluent will inject that (1200) amount of particles every time step of your transient solver. Start and stop-time then will tell Fluent when (by means of computational time of the solver) to start and when to stop injecting particles every time step.
The total weight of all particles is the mass flow rate you choose (in the Set Injections dialog box) times the time step. This will be shown each time step in the Fluent terminal: "Injecting 1200 particles with mass 0.034 at t=0.01".