Many kiltmakers use Gutterman thread, for all the general sewing, it's strong , yet light, yes it's a littel more expensive than normal thread, but it's meant to last and it really is worth using for a garment that will last. I use it for all visible parts including the stabiliser and pad stitching,there is no need to use havy thread for this, heavy thread here adds bulk and doesn't really give a benefit.In professional tailoring the same weight of thread on most of the internal work.
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For the leather work and the steeking I use a linen thread, sometimes called button thread in the UK, it's a little thicker but immensely strong.

The idea of using upholstry thread for the basting is somewhat strange and I certainly wouldn't advise using it, basting is only a temporary stitch and by using a thicker or stronger thread you do run the risk of marking the actual fabric, I use a lighter cotton thread in a contrasting colour, the cotton because it means that the thread will break if under too much pressure so protecting the good fabric, and the contrasting colour because when it is removed any small remains can be safely removed because you know it's not part of the actual sewing of the kilt.

Every kiltmaker doe things differently and there isn't one way of doing it, after a while you will that you prefer one way over another, the only thing I might suggest is to use good quality thread such as Gutterman