There is no doubt that early game you can run one factory out to all 7 cities on a small map. Central location is preferred for this. No trains needed, you can run horse’s and make it work. Just have a good road network.
It is counter productive to have 2 oil wells feed one refinery. However, there are no issues running 3 oil wells to 3 refineries. Not sure where you heard it was a bad idea to run more then one of the same type of factory. But let me assure you, it is false.
My current game I got real real lucky, I have an oil refinery up north, middle and south. I ran the one in the middle until it reached 380 production. Running to all 7 cities, then opened a new one for 2 north cities. When the 1st one reached 350 goods used, I opened the one in the south.
All 3 factories are running great, my cities are having insane growth at the moment. 🙂