Generator Auto Start - When to Stop Cranking

[NorthGuy]

I wonder if it is linear, i.e. the frequency would increase 1Hz for every 60 rpm increase.

Of course. One revolution is one cycle in AC. Some big generators run at 1800 rpm - they produce two
AC cycles per revolution.

After it starts, It may slow down or speed up a bit when the load changes. May even go below 50 Hz when it tries to start a motor, may even die of overload.

If your goal is to protect the loads, then you probably need to monitor both frequency and voltage, which all can be done through a single ADC.

Assuming your generator has grounded neutral and your electronics is grounded too, you don't need a transformer to monitor voltage - a voltage divider will do.