charging iPad Pro 9.7"

While the charging brick which came with iPad Pro 9.7 was 12W, the 18W brick won't cause any harm. It won't likely charge any faster, but it won't hurt the iPad.

You'll need to use a lightning charging cable and charging brick, not the USB C cable and charging brick which came with iPad Pro 11. Do you not have the charging cable and brick which came with the Pro 9.7? The Charging brick which came with the Pro 9.7 doesn't employ USB C.

You could if you want, pick up something like this, and use your iPad Pro 11 brick and cable:

https://www.amazon.com/CONMDEX-Adapter-Converter-Charger-Compatible/dp/B07NQ54YGW/ref=sr_1_3?keywords=usb+c+to+lightning+adapter&qid=1559574289&s=gateway&sr=8-3

Then there's no reason you can't use it on your Pro 9.7.

You should be able to, Yes.

You will need an actual USB-C to Lightning cable. See here: USB-C to Lightning Cable (1 m) - Apple

The cable that came with the 11" iPad is USB-C to USB-C only.