Feca viability late game

Yes, and pretty much all of them, though fire is probably the best on PvM. Personally, I use an omni/wisdom build, and it works very well, especially with a huppermage in my team.

I see, that's wonderful. I've heard they do best if you mix, but what mixes best with int?

Depends on what you want, really. Earth can be hilarious if you have high wis (after all, you can box yourself on top of both glyphs and deal heavy damage while disabling the monsters), water is pretty useful since you can take away range and MP making enemies ineffectual if you want to play the ranged game, and air is the best if you want to tank and lock. I think water is overall better, since you generally don't want to be close to endgame monsters, but all 3 work well. It depends mostly on team composition.

I see, thank you for the help.