I tested this out and here are my results by: Carol
As a lover of crochet as well as embroidery I had both types of thread in my stash. I have to admit I had never thought of using crochet cotton in place of pearl threads. So before answering your question I wanted to try it out, Heather.
I picked the thickest crochet cotton in my box, which was size 10 along with a size 20 for the needleweaving. I worked on a scrap of 22 count Hardanger fabric, starting with a small sample of Kloster blocks.
In hindsight, a finer fabric, such as 28 count evenweave may have given better results.
I found that the size 10 crochet cotton did not cover as well as a size 5 pearl thread. Being tightly spun instead of having the loose twist of the Pearl, it didn’t "spread" and fill in the gaps between the stitches that I would normally see.
I may not be able to describe this as I have in my head, but here goes. Crochet cotton is more of a round thread and doesn’t lay flat on the surface once you stitch with it. This also made it more difficult to thread onto a needle. I ended up using a size 18 tapestry needle but it was quite a job to thread.
I used the size 20 for the needleweaving and was a little happier with this. It gave a less fuzzy effect which worked well. If I did this again I would try a thinner thread, however.
Below is a quick iPhone photo of my resulting sample.
My preference would be to use pearl cotton rather than crochet cotton, but if you pick the right size it is possible to use the latter.