Re the older kilts. I would bet that the pre 1960s kilt would have been sewn with silk. Before poly thread was improved, my grandmother's rule was Linen and cotton fabric get mercerized cotton thread, wool and silk fabric get silk thread.

Some quilters say that poly threads (even today's improved ones) will 'cut' through the threads of the cloth. I still use 100% mercerized cotton (size 50 or 80) at about 12-15 stitches per inch on the hand sewn altar linens that I do for the church.

Now our kilt fabrics are MUCH studier than fair altar linen or quilting muslins, so we should be fine with poly thread. There's always a trade off and since the strength of the poly is worth it to Barb, I bow to her experience.