PCB testing methods during manufacturing and assembly?

All of the soldering I’ve done to this point has been with through-hole components. I hope to move up to some smaller surface-mount parts at some point in the future. I’ve got a Weller WES51 soldering station. There is a large number of ET series tips available. How do I choose the right tip for the components I’ll be working with?

When I audit a supplier, I find that they label PCB bare part. Why do they do that and what is the advantages?

I need to add a resistor into a data line on my project. The data line is a trace and carries low speed data. A through-hole resistor will be fine. The pins for both ends of the trace are exposed, but how do I add the resistor and remove the trace?