I'm playing a mage/thief atm, focusing on 1-handed, light armor, archery and sneak in addition to spells. From what I gather, smithing using the heavy side of the tree (Dwarven, Orc, etc) makes better weapons than the light side. Is this correct?
Yes. Best weapon Light Side will give you is Glass, compared to Heavy Side's Daedric which does 2 more damage on average.
If so, should I focus on that side, even though I'll never wear heavy armor (but Goldie, my follower will).? Which is better?
It's only one perk more and you'll get best light armor in the game anyway(Dragonscale). I say if you have a heavy armor follower, go heavy.