I'm a plane fighter, and is it worth to double class to mage or something else?
If it is, so which kit?
And when? On which level I must do it? Cause as I understand I loose my stats as a fighter untill I reach the same level us mage, is that right?

I should go with a ordinary Mage or why not a Sorcerer. :D

i´d recommend dual-classing at lvl 9 - you´ll get back your fighter abilities quickly enough and can reach the maximum mage level in soa.
incidentally, kensais are better suited for this.