I have on several occasions taken patterns that were intended for worsted weight yarn and done them with smaller yarn in order to shrink the pattern. The smallest I have gone is down to a 1 mm hook with tatting thread. I have always just followed the pattern as is and have gotten good results.
The only way you will be able to tell for sure that it will work out how you want is to make up one of the figures in the crochet cotton and see what size it comes out. I don't know if there is an exact science for figuring out the shrinkage