AC Tacho used for speed measurement
If I am only interested in measuring the speed of the motor, do I only
need to measure the frequency of the tacho signal?

Yes, this will provide a more secure method for inferring speed but, at low speeds the voltage level p-p may be a tad low to adequately detect frequency unless you have provided a signal stage that uses (say) a comparator with a little bit of hysteresis. (or maybe some form of automated gain method).

I don't think I need a rectifier and could use a zero crossing
detector and counter to calculate RPM?

I would recommend that you look at the output on an oscilloscope before making this judgement. You may need to jump through a couple of small hoops at low frequencies and only the visual display will tell you that.