1) Yes, if you play on the PS3/Ps4 you will be playing with people who are also on the PC. Square Enix did not seperate the players, everyone plays together.

2) Yes, the game plays fine using the controller. You can also use a USB keyboard and mouse with the play station. I tried the controller and could not get use to it myself but many play with it no problems.

3) No, at level 30 you will want to level up your secondary class to at least 15 to get your main job. People will expect you to have your job at 30 or shortly after. Some cross skills you will want to have, for example if you are a healer you will want to level THM to 26 to get swiftcast.