or both have separate levels?
or both have separate levels?
Yes they are on Same level. :)
The Jobs are an 'overlay' over the class, as in, they simply modify it slightly and give you access to some abilities while restricting others. They do not have their own level, instead, you remain at your class's level.
I guess the easiest way of saying this is "Class and Job share levels"
last question which gladiator and pug skills useable at level 50 onces when am level 50 warrior?
You can only set 5 skills total from other classes when you're on a job. As long as you've learned the skill from the other class, you're capable of equipping it. You could have a level 30 warrior and have a level 50 pugilist skill (just an example) and equip it, as long as the skill isn't exclusive to the class it came from. Some examples of what a warrior would equip are second wind, featherfoot, flash, sentinel, rampart.