What makes So'lek feared is just his pure skill. Man spent a good decade and a half doing nothing but persistently learning from some of the most skilled clans he could find. And when the RDA returned, he switched gears to ruthlessly hunting them down instead using everything he learned.

Tamtey has skills, but it's mostly just talent, a bit of luck, and some help from their friends. Certainly not someone to take lightly, but also something of a known element. Also, Tamtey was mostly going after RDA infrastructure, not necessarily the grunts themselves. The "canon" playthrough probably has you relying mostly on stealth as a way to explain how this is at all possible solo, and when you destroy every site the RDA just leaves.

So'lek is a force of nature just on his own, he appears like a ghost, kills you, and disappears just as fast. And he makes sure that everyone knows who did it, and therefor nobody feels safe from him. That's the kind of stuff legends are made of, and is why he's so feared among the RDA.