1. No there is no such things rather some of the contracts get paused and start to expire after they finish military
2. They have mandatory service and they can choose (or try to join) different platoons. The first months are like beginners training with the basics and from there whatever your scores are you can choose a platoon. They can even become a trainer for the new enlistments too.

and no thats not the place to ask such things XD there is a forum

1. No, their contract does not expire when joining the military. The contract that idols have with entertainment companies is essentially a regular employment contract. And the government has made sure that when people go to the military to serve the country, companies don't have the right to terminate the employment contract. It would be pretty messed up if employees who leave for their mandatory military service would have to end their job for it. So when idols go to the military, their contract is put on hold, and when they return from the military the contract continues.