I've been considering switching races as of late.
I'm currently Kronyn Builder and I'm finished making plasma planets and I want to switch to another race.

I have Mylarai, Genetarr, and Zolazin unlocked.
Which race should I choose, as well as which profession within each?

None of them, really. Konqul or Taltherian (especially Taltherian) would be much better for you. But, if you are determined to pick one of those 3:

Mylarai - Fixer - Always useful in a pinch, both attacking, defending, PvP, bases and NPCing

Genetarr Fixer - Same reason

Zolazin - Saboteur - Valuable to larger legions. Only real reason to pick Zolazin

After I get sehrin, I'll be starting to work on the talth mission line. But which one of Mylarai, Genetarr and Zolazin should I pick...?

Konqul bests them all, IMHO. If you're short on mining, you could sell the 17x Plasmas and buy 2 15x Toxics as Kronyn, so Mylarai isn't worth while.