Which SF to use and if interference will affect me

I am setting up a LoRa network with approx 100 nodes in a farm which is approximately 6x12 KM in size. I need to track the nodes and hence they will send their GPS coordinates once every minute or so. I plan on placing approximately 10 gateways. My question is can I use any SF or only look at SF12 and maybe SF11. Also, if two or more nodes transmit at the same time will it affect reception of the packets? If it does, I was thinking of implementing a polling system wherein the gateway might poll individual nodes for their coordinates. Will this work or should I look at other alternatives? Also, can I reduce the number of gateways in any way?

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