Is there any problem on using a fast charger to charge my iPhone 6s?

No problem, provided you use a genuine Apple cable and a certified USB source between 1 and 2.5 amps, and any apple USB adapter, automobile or laptop USB will be OK.

The iphone will regulate the charge current, but a rogue foreign USB source with too high a voltage can cause overheating.

gilda260 wrote:

What about using a non-Apple wireless charger, could this damage my Apple device?

Since the iPhone 6s does not support wireless charging, this question is pointless.

Also, apple does not make a wireless charger at all at this point.

This shouldn't be a problem, providing you are using a genuine Apple cable and USB wall plug.