No, cutting a clematis back will never hurt it regardless of the type. It may delay flowering for a season depending on your growing zone and the pruning class, but it will also stimulate more lush and exuberant growth in the spring. You can often tell what pruning class a clematis belongs to by when it blooms. If it tends to bloom early on in the season on older wood, it will most likely be a type II or type I. If it blooms on new growth later in the season, then it more than likely will be a type III. The one caveat to all this is determining what is early or late in your zone. The further south you go, the earlier all types of clematis will bloom compared to those further north.

Any pruning is going to give you more vines and flowers so pruning an unknown as a III is better than not pruning at all.