Both of us are making deliveries using trucks, although he is using more number of trucks. Does that have anything to do with it?
No, number of trucks serving the line do not make any difference in one payment. However distance does make a difference and you said that rival was hauling coal from farther away. That is reason why he gets more money. However Alberth said that speed of delivery affect payment, but coal dosen't take too big hit on long delivery time-
Also, once I've started providing iron to a steel industry, how do I provide steel to other places?
If I remember correctly factory should appect steel and food items to produce goods. You provide them by running steel truck or train with steel wagons between industries.
And I'm also providing coal to a power generating industry and I'm getting income but when I click on the power generating industry, it says "Requires:Coal" even though I am satisfying that requirement.
Powerplant dosen't produce anything and that Requires:Coal is only there to tell you what it needs.