Hi everyone, question: if I wish to have a good mage\fighter should I go for xivilai? It seems to me that they are surely stronger than dark elf. Am I wrong?

Xivilai are bad asses. It's one of my favorite options. Lots of back story possibilities that make it interesting, whether you are aiming for good or evil. Definitely a great combo for fighter/mage. I recommend the atronach as a birth sign to make it more challenging. Also, I recommend Arthmoor's Alternative Beginnings to give you lots of starting flexibility for your character concept.

Well, for Fighter/Mage characters, it is hard to do better then a Xivilai. Good Stats for it, powers which fit for it(awesome bound weapon power which gets better if you use the LAME Glue file, and a clanfear summon), the largest magicka bonus one can get for races(starts at 40, hits 400 end game IIRC), and they're big and scary.

As for it as a RP choice, that depends on if you want them to be obviously good, or struggling. It's really a personal choice when it comes to RPing. Personally I play a Xivilai because they are large, hit hard, and can cast the most spells out of the base races. Sea Elves however, are my personal favorite, since they can make spells on their own. Not much good for Melee, but that suits me just fine.