Charging a battery usually creates some heat. Charging it faster would probably create more heat. Apple says right on their website that the iPad charger works with the iPhone, so I wouldn't worry about it.
+1 I think but can not confirm the charge seems better with the iPad charger, not sure if it does it quicker or something but does seem to charge faster.
I charged my iPhone 6 with the standard 5W adapter for the first two or three charge cycles. I've been using my iPads charger ever since. 24% to 73% in half an hour is handy when I need a quick charge before a shift.