Some servos respond differently to the input square wave wavelength. If you look up spec sheets of the servo, you will see something like:
"Clockwise/Pulse Traveling 1500 to 1900usec"
If your servo tester only goes to say 1700usec, then your servo wont rotate the full direction. You basically want the servo tester range to be greater than the servo range.
So you could consider getting another servo that works in the range of your servo tester (check datasheets), or you can attempt to hand modify the servo tester.
To do this, find the capacitor inside the device, and change it to another. If you place a second capacitor in parallel with the current one, this will increase capacitance and hence slow the timer down (increasing wavelength). Reducing the capacitance will increase the timer speed. But I really couldnt tell you how well this will increase the range of the tester since I suck at R/C circuits . . .
Programming the microcontroller to ramp down/up a servo speed is easily possible. You just give it a list of speeds and tell it the time delay to wait before it goes to the next speed on the list.
However, after you modify a servo, it loses position feedback control. This basically means that the servo angle will slowly drift as it rotates with no way to correct for it. It might rotate 19 times, or maybe 21 . . . of course if your situation is very predictable you can tweak the code until its satisfactorily close.
As for 75rpm . . . that is a little fast for a servo, and too fast for some. Check the spec sheets just to make sure. You will see something that says '0.15sec/60 degrees at no load' . . . which means .9sec/360, or 66.7rpm.
60sec/(.15sec*6) = 66.7rpm
Digital servos are generally faster.