Faction quests...

Well i completed all the main quests in factions, and most of the side quests, i got 1 quest left in quest log in jade sea, its the only one i could fine, and i got 2 quests to explore territories. Should i do the kurzick quests or thats not really possible or useless since im luxon?

You can do all quests does not matter what faction you are with.
The ideal way to do it, is do one side and once done do the other after you cash in your faction if you want armor and such.

k, one more question, i made a ranger and i dont wanna do all the quests like 30 in kaineng city, is that a problem or no?

Nope, you can skip them, since side quests are optional its just nice to do them to get the xp and the bit of cash they give.

If for example you want luxon armor with your old character and also your new ranger, then you can wait on doing the kurzick quests to keep your luxon faction so your ranger can use that faction to talk to the merchants. This way you donot have to redo those quests if you dont want to as well.