Any recommendations for how my mountain dragons should look?

Think stone. So rocky looking ridges on the back, stony grey colors, scales that look like shale, etc.. Claws and teeth that look like shards of rock. Maybe obsidian coloration if you're thinking more fire mountains.

Well, I suppose it depends on the world you're building, but personally, I would say the 'tradition' is usually to find them very young and train/tame them then. A dragon with rocky features would lay eggs that look a lot like big, ovoid chunks of rock. Maybe that's a camoflage defense, so the eggs can't be easily found and stolen. But young dragonlings would be easier to tame than fully grown ones.