While I am no expert, I can add some observations. Late in the GP9 era (about 1957) EMD switched from 36" fans (small) to 48" fans (larger). As you have noticed, some models have a mixture of both sizes, and rebuildings have further muddied the waters.

Up thru the GP20 (???) the units had radiators at both the front and back of the engine room. EMD apparently decided that having radiators above the main generator was not a good idea so the GP30 and everything since had a larger single radiator section in the rear of the unit. The inertial air filters occupy the forward section.

I believe that you are correct that temperature sensors trigger the fans, and I think the fans have at least two operational speeds. I suspect that these temperature thermostats are set at different temperatures to trigger fan activation at different times.

The GP7 that I am most familiar with, is former BN 1617, built Dec 1953, right at the end of GP7 production. Almost 60 years later it is mechanically in it's as built configuration and it has 36" fans; two on each radiator section, and the front one on the rear has a winterization hatch.

The few times I have been topside, I have observed that the forward fans on the front radiator section seem to be running but the rear fans are not. You can hear the forward fans turn on/off in the cab. Sometime I will have to check it out when we have been working the unit hard and see what the fan status is.

Geeps with dynamic brakes (1617 does not have dynamics) had an additional 1 or 2 fans in the center to cool the dynamic brake grids.

Hope this helps.