What I said is that if they were on the reference then they'd be okay, not the opposite, sorry if I explained it weirdly.

About the first question, no, you can't use different pens. The only exception is using a brush pen or a marker to fill big shadow shapes more quickly, but other stuff like that, such as lineweight needs to be done all with the same pen.

About adding lineweight, yeah, it needs to be added with ghosting and prioritizing confidence over accuracy, just like in all the other exercises.

And the last one, it's better if you do it after you draw all the forms, because you might be drawing cast shadows in a place that could after be covered by other sausage, covering the cast shadow. And similarly with lineweight, you might be adding lineweight to some part that could end up not being visible to the viewer depending on how you continue de exercise.

And about the exercise itself, nice improvement! Just one thing to keep in mind, try to keep the sausages roughly the same size, don't do some big and others really small.