Is that really necessary due to the fact that the motors will never spin 360 degrees in one time-lapse.

Yes, it is necessary to know how many steps you want to take, somehow. That can be total number of steps or an angle and you calculate the total number of steps knowing how many steps equals 360 degrees.

Could rpm be brought into this? I really have no idea.

Yes, but speed is not a stepper motor's strong suit. Repeatability and accuracy of step size are.

?

You've go to say "go" somehow...

Could I specify the interval in between steps, then I wouldn't have to sepecify a total time for the shoot?

I'm leery of customers asking if they can specify the requirements in some particular way. It sounds like they really don't know what they want. If there is an equation in the form A = B + C, and you know two values (A and B, A and C, or B and C) you can calculate the other one. You have an equation involving number of steps, interval between steps, and total time. Specify any two, and the program can compute the other one. An overly-constrained situation arises when you want to specify all three, and the third value doesn't fit the equation.