There are quite a few of us on this forum that recommend cutting all clematis back severely, especially in the first few years to encourage bushier growth and less susceptibility to wilt for the type IIs. Since the base of the plant is where all strength and nutrients flow from , it would only make sense, to me anyway, to cut them back severely if they look fragile. A fragile base is not going to support luxurious top growth. Clematis will not be hurt by pruning them back. The only thing it will do is postpone until later in the season when a clematis will flower. At worst, you might you lose one season of flowers. In the long run however, the plant will be better off for it.

Yes, you should prune them down hard. I would do it lower than 18" even for group B/II. I put up with spindly weak few flowering vines for years until I got mad and whacked them. They all responded wonderfully, even the group A/Is.